After the first two rounds of Democratic presidential debates earlier this summer — which saw back-to-back nights of 10 candidates each — tonight’s ABC programming will offer the most pared-down debate so far this cycle.

This third round in Houston will be a single night with 10 candidates on stage after the Democratic National Committee doubled the polling and individual donor thresholds required to make the cut.

Sixteen Democrats remain in the race, while some once-hopefuls — such as U.S. Sen. Kirsten Gillibrand of New York and former Colorado Gov. John Hickenlooper — have dropped out.

Dante Scala, a political science professor at the University of New Hampshire in Durham, offered “professor’s notes” on what to look out for tonight, as the candidates go head-to-head for three hours.

Order reflects how candidates will appear on stage alternating from left to right, starting with those polling lowest nationally in the wings down to the highest polling in the middle. (New Hampshire polling averages are from the RealClear politics algorithm.)

Amy Klobuchar

Background: The senior U.S. senator from Minnesota, Klobuchar was previously the Hennepin County attorney before becoming the first woman elected to one of Minnesota’s Senate seats.

Visits to the region: Four, an April visit to Peterborough’s Waterhouse Restaurant, a May speech as the keynote speaker of the Cheshire County Democrats’ annual spaghetti dinner in Keene, an August stop in Greenfield for the Hillsborough County Democrats’ Summer Picnic and a business roundtable followed by a Q&A town hall at Stonewall Farm in Keene.

N.H. polling average: 1.6 percent.

What’s changed: After rolling out her 137 bullet-point agenda of priorities for her potential first 100 days in the White House in June, Klobuchar has made plans to open a Keene field office, released a plan for rural America and expanded her ground game in the Granite State. Klobuchar appeared on The Sentinel’s politics podcast, Pod Free or Die, in August.

Professor’s note from Scala: “She needs to present herself as a moderate alternative, or a potential Joe Biden replacement, more assertively ... She doesn’t necessarily need to go after Biden, but she needs to start drawing more of a contrast between herself and the more progressive candidates, and explain how someone who is moderately liberal like herself, why she would be the better nominee for the party.”

Julián Castro

Background: Former secretary of Housing and Urban Development under President Barack Obama and former mayor of San Antonio.

Visits to the region: Two, an April visit to Keene State and an August stop in Greenfield for the Hillsborough County Democrats’ picnic.

N.H. polling average: 0.6 percent.

What’s changed: After having a breakout moment in the first debate, Castro has remained stagnant in the polls as he focuses on his main policy areas of immigration and affordable housing, the latter plan drawing praise from local advocates for its attention to rural housing.

Professor’s note: “Castro is trying to find some leverage or a toehold in this race. I think he needs to start explaining the importance of Latinos and Latinas to the Democratic coalition.”

Cory Booker

Background: Junior U.S. senator from New Jersey and formerly the mayor of Newark. A Rhodes scholar and graduate of Stanford University — where he played tight end on the football team — Booker’s background has been in community organizing since attending law school at Yale.

Visits to the region: Three, two as a declared candidate in May at a house party in Keene and in August at the Hillsborough County Democrats’ picnic, and previously during his exploratory-committee phase in December, with another house party in the Elm City.

N.H. polling average: 1.6 percent.

What’s changed: Booker has kept championing his baby bonds proposal, which he discussed with The Sentinel on Pod Free or Die, and recently rolled out a climate change proposal. His inspirational message and ground game drew praise from local party devotees at the N.H. Democratic Party Convention last weekend in Manchester.

Professor’s note: “He needs to get out of the shadow of [Elizabeth] Warren and [Bernie] Sanders, and present himself as someone who can be electable like Biden, but a younger, more vibrant version of Biden.”

Beto O’Rourke

Background: A former three-term congressman from Texas and former member of the El Paso City Council, O’Rourke rose to national prominence in his unsuccessful bid to unseat Republican U.S. Sen. Ted Cruz.

Visits to the region: Three, a town hall event at Keene State in March, a July anti-child separation protest rally in Peterborough, and a return to Keene State last week.

N.H. polling average: 1.2 percent.

What’s changed: After the Aug. 3 shooting in his hometown of El Paso, O’Rourke took a reprieve from the trail and returned with a more fervent message on gun control, proposing a mandatory buyback of assault weapons. At Keene State last week, he told The Sentinel he would not send federal agents to retrieve the firearms.

Professor’s note: “Well, the question I have about Beto is: Does he really think he can be the nominee of the Democratic Party? ... If the answer to that is no, then I guess he’s in it to deliver a message about guns, about white nationalism ... and almost being the conscience of the debate.”

Pete Buttigieg

Background: Mayor of South Bend, Ind.; served in Iraq as an intelligence officer in the Navy. Buttigieg is a former Rhodes scholar and would be the first openly gay president.

Visits to the region: Three, once in February at the Orchard School and Community Center in Alstead before declaring his candidacy, then as an official candidate in May at Keene High School and an August house party in Hancock.

N.H. polling average: 7.6 percent.

What’s changed: After emerging from obscurity and solidifying a spot in the top tier of polling candidates, Buttigieg has debuted a slew of policy proposals, drawing praise locally for his rural health care and rural economy plans. He also opened a Keene field office earlier this month and released a climate change plan.

Professor’s note: “I think at this point, he’s got to be thinking about how — in a positive way — to start building bridges to other parts of the Democratic Party besides the niche that he’s found, I think especially with kind of working-class, blue-collar white voters.”

Andrew Yang

Background: Tech entrepreneur and philanthropist from New York.

Visits to the region: Four, last year in June at the Hannah Grimes Center for Entrepreneurship in Keene and this February at The Works Café in the Elm City before a stop at Post and Beam Brewing in Peterborough. Yang was also the first guest on Pod Free or Die, and more recently attended the Hillsborough County Democrats’ picnic and made a a late-August visit to Keene State.

N.H. polling average: 2.8 percent.

What’s changed: Yang’s fervent online following, dubbed the #YangGang, has turned out in person at events, including at Yang’s recent Keene State stop. He has been steadily gaining in the polls, and his campaign promises he will do something on stage tonight “no presidential candidate has ever done before in history,” in a cryptic message to The Daily Beast.

Professor’s note: “You know, Yang’s just gotta be Yang. I think he had a good second debate, I think he knows what he’s about in terms of delivering his particular message on [universal basic income] ... He can be the person who focuses on technology and innovative policies for the next century.”

Bernie Sanders

Background: Junior U.S. senator from Vermont, formerly the at-large congressman for Vermont and mayor of Burlington; runner-up in the 2016 Democratic presidential primary; remains an independent but caucuses with the Democrats in the Senate.

Visits to the region: Two, a full-house rally at The Colonial Theatre in Keene in March and another packed rally at the Peterborough Town House in August.

N.H. polling average: 21 percent.

What’s changed: Not much, though voters in Peterborough noted a slight shift in his rhetoric to more personal stories instead of abstract policy lectures. Sanders also gave his first one-on-one interview with The Sentinel at the convention.

Professor’s note: “I think Sanders — now that he’s got Biden [alongside him] on the stage — I think he goes after him. I think he, unlike Warren, may not have as much to lose by going aggressively after Biden. So I think he does kind of a reprise of what he did with Hillary Clinton [in 2016], and I think we could see some fireworks.”

Kamala Harris

Background: Junior U.S. senator from California; former California attorney general and former San Francisco district attorney.

Visits to the region: One, an April town hall at Keene State College, which required an overflow room.

N.H. polling average: 7.2 percent.

What’s changed: After seeing a surge in the polls after the first debate, Harris is more or less where she started beforehand after being put on the defensive over her health care plan and prosecutorial record in the second debate. She has launched a Keene field office, and was on the shortlist of some local Democrats at the convention.

Professor’s note: “I think what she has to be concerned about is the [moderators] will be throwing questions at her about her viability, so she’s gotta find a way to blow past that, and again, try to find a way to square up against Biden or perhaps against Warren and make her case again. She needs a moment, because it’s been a long time between moments for her.”

Elizabeth Warren

Background: Senior U.S. senator from Massachusetts; previously a Harvard Law School professor of bankruptcy law and adviser to federal oversight programs.

Visits to the region: Two, a Keene State College town hall in April and a packed rally at the Peterborough Town House in July.

N.H. polling average: 18.2 percent.

What’s changed: A lot. Warren has had steady, almost uninterrupted momentum in polling since the first two debates, and drew the loudest applause of any candidate at the convention. Her large crowds and long line of policy proposals have vaulted her into the top tier.

Professor’s note: “I think Warren has to be prepared this time for, perhaps, being on the defensive for the first time. I’m not counting John Delaney going after her [at the first debate], but actually facing — probably from Biden, maybe from Harris, maybe from Sanders, or, you know, one of the other moderates — but facing more of an affront, given that her press and her media has been so good over the past month, that she may face some sort of attack from a major player.”

Joe Biden

Background: Former vice president; ran for president in 1988 and 2008 and was the U.S. senator from Delaware from 1973-2009.

Visits to the region: One, a Keene State rally in late August.

N.H. polling average: 21.4 percent.

What’s changed: In polling, Biden has remained the front-runner, but has come under increased media scrutiny over gaffes, as well as attacks from his opponents over his lengthy record in Washington. Biden also appeared on Pod Free or Die after the Keene State rally.

Professor’s note: “Well, he’s basically on probation, I think — on watch — this debate, every debate going forward, in the sense that I think he’s kind of stuck in a cycle where the press is going to be watching him most of all for any kind of gaffe, mistake, signs of old age — whatever you want to call it.”

Jake Lahut can be reached at 352-1234, extension 1435, or Follow him on Twitter @JakeLahut.