WASHINGTON — Rivals eager to dislodge Joe Biden from his front-runner's perch tried attacking him in last month's debate and going easy on him in Thursday's debate. Neither worked.

The night in Houston highlighted a puzzle that has vexed 2020 hopefuls — how to knock Biden off his game. Frequent gaffes haven't hurt him. His 76 years haven't stopped him — and on Thursday, it was Bernie Sanders, 78, who was hoarse and hunched. His checkered history on race hasn't sunk him.

And in a Democratic debate in which Barack Obama was revered, his former vice president was well-positioned to claim he's the rightful heir.

In the end, the nearly three-hour debate seemed unlikely to reshape the current contours of the Democratic primary: Biden in a persistent lead as a rising Elizabeth Warren and a steady Sanders battle for second place. None of the others polls in double-digit support, with Kamala Harris and Pete Buttigieg rounding out the top five.

"I do think that candidates have seen over the course not just in the debates, but over the course of this campaign, that attacking Joe Biden is not the way to advance yourself in the polls," Biden deputy campaign manager Kate Bedingfield told reporters after the debate.

Ruben Gallego, an Arizona congressman who has endorsed Harris, argued that the dynamics of the race will change once the field narrows.

"Right now, asking a voter to dislodge what are positive feelings from Biden — because he was a good vice president — for nine other people that they may not know as well yet is going to be very difficult," Gallego said after the debate. "I think once you're talking about five people, then there's going to be a different ballgame."

Biden's age showed through not just his gray hair but his dated references. His first line of the debate evoked President John F. Kennedy, who was elected in 1960. Without irony, he called on families to use a "record player," a format that largely fell by the wayside long before many of today's voters were born.

A much-anticipated clash between Biden and Warren, who represent opposite visions for the party, failed to materialize. Harris didn't attempt another assault on Biden after it backfired in the previous debate. Sanders politely jabbed him for voting for the Iraq war, but Biden had already said it was a mistake. Julian Castro, a rare aggressor on Biden, questioned his short-term memory in a jab that fell flat because Castro flubbed the facts.

Biden set the tone early by tying himself — yet again — to Obama, an alliance that has powered him to a commanding lead with the black community, a crucial voting bloc in the Democratic primaries and one that both Sanders and Warren are struggling to reach.

In the first back-and-forth of the debate, Biden contrasted Warren's support for the Sanders government-run healthcare plan with his own proposal that builds on Obamacare. "The senator says she's for Bernie," he said of Warren. "Well, I'm for Barack."

That forced Warren to play on his terrain. She said, nodding toward Biden, "We all owe a huge debt to President Obama, who fundamentally transformed healthcare in America."

Biden was grilled twice by moderator Jorge Ramos of Univision about one of Obama's most unpopular decisions: to deport record numbers of undocumented immigrants during his first term. Biden stood firm in his allegiance.

"We didn't lock people up in cages, we didn't separate families," he said, adding that Obama created the program that allowed people brought to the U.S. illegally as children to remain in the country as adults. "I'm proud to have served with him," he added. "The president did the best thing that was able to be done at the time.

Biden later cited one disagreement with Obama — when he said he opposed the troop surge in Afghanistan.

Other candidates had some strong moments.

An energetic Beto O'Rourke lit up the crowd as he declared, "Hell yes, we're going to take your AR-15s, your AK-47s." Warren drew cheers for proposing to abolish the Senate filibuster to achieve gun control, and spoke eloquently about trade. Harris repeatedly pivoted to attacking President Donald Trump, eliciting approving laughter by calling him "a really small dude" in an allusion to the Wizard of Oz. Pete Buttigieg evoked his military service to call for preventing needless wars.

But if there's one thing that's clear, five months before voting begins, it's that Biden's support in the party runs deep enough to withstand his litany of unforced errors.

"People are comfortable with Joe," said Democratic Rep. Sheila Jackson Lee, who represents Houston and is neutral in the race. "The other candidates are saying, give us a chance, and they did a mighty good job. But I think the real issue is comfort."

"African American voters want to make sure that they have a proven entity. You can prove yourself, but they like a proven entity as well," she said.

Bloomberg News