If Joseph R. Biden Jr. and Elizabeth Warren are headed for a showdown, neither of them appears in a hurry to get there.
The two candidates have seemed to be on a collision course for much of the last few months: Mr. Biden as the Democratic front-runner and de facto leader of the party’s moderate wing, with a steady but hardly dominant lead in polls, and Ms. Warren as his rising challenger, slowly trimming his lead and perhaps surpassing Senator Bernie Sanders of Vermont as the focal point of progressive energy in the race.
Despite some pre-debate chest-thumping by Mr. Biden’s camp, no great clash occurred in Thursday’s debate in Houston, the first time he and Ms. Warren have been onstage together during the primary.
It is almost inevitable that the race will grow more combative, and in the wake of the debate there were signs that some of the leading candidates were ready for conflict. Mr. Biden, who only challenged Ms. Warren in a single exchange on health care Thursday, delivered a veiled swipe at her policy-heavy campaign at a fund-raiser on Friday, saying that the country needed not just plans but also “someone who can execute a plan.” And Mr. Sanders sought to reignite his own clash with Mr. Biden, releasing a statement from his campaign manager that accused Mr. Biden of “echoing the deceptions and falsehoods of the health care industry.”
Ms. Warren, for her part, ignored the back-and-forth, seemingly content with a debate performance that her campaign said had presented her “like a president.”
Supporters of Mr. Biden claimed encouragement from what they called the best of his three debates, but his tendency to garble his words, and his dated instincts on sensitive matters of culture and race, are sure to be tested even more strenuously in the coming months.
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Ms. Warren, the Massachusetts senator, and Mr. Sanders are plainly unintimidated by Mr. Biden’s lead in the polls. While he has a solid electoral base, made up chiefly of moderates, older voters and African-Americans, he has not gained new support since he entered the race, and now appears vulnerable to defeat in at least three of the four early primary and caucus states — Iowa, New Hampshire and Nevada.
“The question is, who is trying to play a long game, against Biden and for the nomination,” said Dan Sena, a strategist who helped oversee the Democratic takeover of the House last year.
Mr. Sena, who is now advising Senator Michael Bennet of Colorado, a presidential candidate who failed to qualify for the Houston debate, suggested that Ms. Warren appeared satisfied with pursuing her own gradual and disciplined strategy.
“My suspicion,” he said, “is that the Warren campaign in particular is looking at a much, much longer runway.”
Among Ms. Warren’s goals in the near term are to consolidate her backing from liberals and expand her appeal to lower-income voters and minorities. Attacking Mr. Biden might not serve either goal. In the debate, Ms. Warren spent far more time highlighting her upbringing in a working-class Oklahoma family than engaging on any level with Mr. Biden. She must also still navigate the enduring presence of Mr. Sanders on the left.
And Mr. Biden may have more immediate challenges. On Thursday night, he repeatedly expressed himself imprecisely — saying at one point that no nonviolent criminals should be in jail, when he meant to refer only to nonviolent drug offenders. He also referred to himself as the vice president of the United States, without appending the modifier “former.”
In a moment that drew criticism after the debate, Mr. Biden responded to a question about the legacy of slavery with a meandering answer that wound up involving a recommendation to place social workers in the homes of parents who “don’t quite know what to do.” Those parents, he suggested, might do well to “make sure you have the record player on at night” so that young children grow up hearing more words — a suggestion he has made more broadly at other times, though he does not typically allude to that particular technology.
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Mr. Biden’s obvious unsteadiness at certain moments opened the way for other candidates to question his strength as a challenger to President Trump. Senator Cory Booker of New Jersey, appearing on CNN after the debate, noted that Mr. Biden “tends to go on sometimes,” and depicted him as out of touch.
“At one point, he’s talking about people in communities like mine listening to record players — I don’t remember the last time I saw a record player,” Mr. Booker said, adding, “There are definitely moments when you’re listening to Joe Biden and you just wonder.”
Mr. Biden and his campaign surrogates have taken umbrage at suggestions that he has slipped in his political acuity, and on Friday they pushed back particularly hard on Julián Castro, the former federal housing secretary, for having bitingly questioned Mr. Biden’s recall of his own policies in the debate. Mr. Castro, who is polling near the bottom of the field, was the only candidate to attack Mr. Biden in such strong terms, drawing backlash that illustrated why other candidates had shunned that approach.
Asked by a reporter Friday whether he would release his medical records to address “concerns,” Mr. Biden, 76, said he would do so before votes are cast.
“What the hell concerns, man, you want to wrestle?” he said lightheartedly. He continued, “When I get the next physical. Look, I’ll release my — before there’s a first vote, that’s — I’ll release my medical records. There’s no, I mean there’s no reason for me not to release my medical records.’’
In Mr. Biden’s camp, there is a persistent sense that his rivals and the news media are underestimating him and giving him too little credit for the blocs of support he has already claimed. Whatever the flaws in Mr. Biden’s performance in Houston, there was no exchange in which an opponent obviously routed him, as Senator Kamala Harris of California did in the first debate and Mr. Booker did in the second.
He has faced many controversies throughout the campaign, his allies note, and he is still in the lead — a reflection, they argue, of the good will he enjoys from rank-and-file Democrats who feel that they already know him and who see him as best positioned to defeat Mr. Trump.
Still, Mr. Biden may have exposed his anxieties about Ms. Warren in the one debate exchange that pitted him against her. He used his very first answer of the night to chide Ms. Warren for calling for the replacement of the Affordable Care Act with a “Medicare for All”-style system. Ms. Warren declined to swipe back; instead, she praised the A.C.A. but said it could be improved upon.
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Mr. Biden never went after Ms. Warren again. It was a more restrained performance than what several of his campaign surrogates had seemed to forecast, suggesting that he would question the pragmatism of Ms. Warren’s policies.
Instead, the differences that emerged between Mr. Biden and Ms. Warren arose largely by implication: the side-by-side contrasts between Mr. Biden’s manner of speech and Ms. Warren’s crisp, detailed answers; or between his proud ownership of the ideological middle and her refusal to be outflanked by other liberals.
Allies of Mr. Biden pointed to the exchange over health care as an example of how he would draw contrasts with Ms. Warren and other candidates without becoming venomous.
“He made clear that everybody knew about the differences in their two health care plans,” said Representative Cedric Richmond, a Louisiana Democrat and Mr. Biden’s campaign co-chairman. Asked about Mr. Biden’s aversion to making personal attacks onstage, Mr. Richmond said: “It’s not his style, he would never do that.”
Indeed, Mr. Biden, who served for decades as a senator from Delaware, at times acted more like a man on the Senate floor than someone in a crowded presidential field, referring to Ms. Warren as his “distinguished friend” as he raised questions about her plan to pay for single-payer health care.
“There’s no pressure at this point for either one of them to go on the attack,” said Mark Mellman, a Democratic pollster. “The potential downside is, frankly, much more likely than the upside.”
The relative lack of engagement between Mr. Biden and Ms. Warren may carry risks, too. For Mr. Biden, there is the chance that holding back against Ms. Warren could allow her to amass momentum that would make her harder to overcome later on. And for Ms. Warren, there is the possibility that her status as the challenger who is creating the most buzz could pass to a rival candidate — perhaps someone more eager to make the case against Mr. Biden directly.
Kathy Sullivan, a former head of the New Hampshire Democratic Party, said it would be a mistake to write off the rest of the field in favor of the top two or three candidates.
“I know people still want to focus on Sanders and Biden and Warren and say it’s a three-person race,” she said. “I don’t think that’s accurate. I think it’s a mistake and it would be unfortunate if people didn’t give all of these other candidates a good look, too.”