Democratic presidential candidate Hillary Clinton and Bronx borough President Ruben Diaz Jr. rode the subway from 161st Street to 170th Street in the Bronx Thursday. (Yana Paskova for The Washington Post)

The contest between Hillary Clinton and Bernie Sanders grew increasingly nasty Thursday with a series of testy exchanges that have prompted widespread concern among Democrats that their rivalry is doing lasting damage to the party and the eventual nominee.

With both candidates launching 10-day sprints here ahead of New York’s April 19 primary, the strain and resentment of a hard-fought and unexpectedly long contest boiled over repeatedly in interviews, speeches and other public appearances. The senator from Vermont refused to retract his assertion that Clinton is not qualified to be president. Clinton dismissed that claim as “silly” and countered that Sanders has repeatedly made promises he can’t keep.

In an interview with The Washington Post on Thursday, Sanders stood by his view that Clinton is not qualified — but he also pledged to support her if she is the nominee.

“Look, as I’ve said before, on her worst day, she is 100 times better than Donald Trump or Ted Cruz or the other candidates,” he said. “To me, that is not a very hard choice.”

Bernie Sanders said Hillary Clinton is not "qualified" to be president because of the donations she accepts from the fossil fuel industry, while Clinton questioned whether Sanders's ideas are realistic. (Peter Stevenson/The Washington Post)

Sanders continued to blame Clinton for going on the attack and said he has simply been defending himself. And while he expressed regret for the tenor of the campaign over the previous 24 hours and said the acrimony will make it harder for Democrats to unite in the fall, he also said he does not regret his own statements.

“When somebody says that I am unqualified to be president and gives her reasoning,” Sanders said, “I think it is totally appropriate for me to respond as to why I think she may not be qualified as well. And that has to do with her views and her actions on a number of the major issues facing this country, and the way she’s run this campaign in terms of how she’s raised her money.”

Clinton had raised questions in a television interview about whether Sanders was prepared to be president, but she repeatedly stopped short of saying he was unqualified.

Some Democrats are worried about potentially longer-term fallout of an increasingly personal conflict between Sanders and Clinton. Most of those Democrats are Clinton supporters who view her eventual nomination as inevitable despite the drawn-out nomination battle with Sanders. And most blame him for the ugliness.

President Obama, who has sought to stay out his party’s nominating contest, weighed in Thursday though a spokesman. Traveling with Obama on Air Force One, White House spokesman Eric Schultz said Obama believes that Clinton “comes to the race with more experience than any non-vice president” in recent campaign history. Schultz emphasized that Obama feels “fortunate” that Clinton, whom he defeated in a sometimes nasty battle for the 2008 nomination, served as his secretary of state.

Sanders based his assertion about Clinton’s lack of qualifications on claims that she is too closely tied to Wall Street, a charge he has been repeating for months. He also said her candidacy was undermined by her support of the Iraq War and her backing of what he termed a series of “disastrous” trade deals.

Democratic presidential candidate Bernie Sanders spoke at the Pennsylvania AFL-CIO convention in Philadelphia Thursday morning. (Mark Kauzlarich/Reuters)

“There are policy disagreements he may have with her on some things — let’s stick to those, let’s not say that the most qualified candidate for president is simply unqualified,” said Sen. Claire McCaskill (D-Mo.). “We’ve got to refrain from ad hominem attacks . . . we’ve got to stay focused on what we’ve got to do in November.”

“It concerns me deeply,” said Sen. Dianne Feinstein (D-Calif.). “What he does is divide the Democratic faithful, and why would he want to do that?”

Some Democrats said they found Sanders’s words particularly troubling because, outside the heat of the campaign trail, they don’t really think he means them.

“I really don’t think he believes that,” said Sen. Timothy M. Kaine (D-Va.), another Clinton supporter. “Nothing he’s ever said to us had conveyed that sentiment. Competition’s tough. I hope that they might back off it a little bit.”

Others were more sympathetic to Sanders.

“I think in both cases, you saw over the last day frustration and fatigue in the rough-and-tumble world of New York politics,” said former Wisconsin lieutenant governor Barbara Lawton, a Sanders supporter. “I think the fatigue and frustration will give way to the dignity they both have. I have every confidence we’ll settle into something we can be more proud of.”

Clinton said in an NBC interview: “I think it’s kind of a silly statement. But he’s free to say whatever he chooses.” Her husband, former president Bill Clinton, and a large cast of other supporters make public and sometimes indignant defenses of her credentials. Her campaign produced a cheeky online true-or-false quiz to bolster the argument that her long résumé and practical experience make her eminently qualified.

Thursday was a busy day for both candidates. Sanders addressed the Pennsylvania AFL-CIO in Philadelphia in the morning before heading to New York, where he taped an appearance on NBC’s “Late Night With Seth Meyers.”

Clinton held a brief news conference in the Bronx and took a made-for-the-cameras subway ride before attending fundraising parties the rest of the day. Her campaign made sure to note that she knew how to operate an automated fare card for her one-stop ride — a poke at Sanders and his reference in a tabloid interview this week to subway tokens, which the system no longer uses.

Clinton noted, laughing, that the system switched to fare cards years ago — during her first term as a senator representing New York.

Her campaign also sought to take advantage of the “unqualified” flap by using it to solicit donations.

“This is a ridiculous and irresponsible attack for someone to make — not just against the person who is almost certainly going to be the nominee of their party this November, but against someone who is one of the most qualified people to run for the presidency in the HISTORY OF THE UNITED STATES,” Clinton spokeswoman Christina Reynolds wrote in a fundraising email to supporters Thursday.

The Clinton campaign has been eager to knock Sanders’s halo off by emphasizing what it has described as the increasingly negative and political nature of his campaign. Trying to convince Democrats that Sanders is more of a typical politician than a principled crusader has been an ongoing — and largely unsuccessful — effort. But the latest skirmish provided a new chance to try.

Sanders may have given his rival a boost by attacking Clinton in a way that seems to cut against his issues-only ethos, said Dan Pfeiffer, a former top aide to President Obama.

“Sanders made a strategic mistake by going down this road, because it’s off-brand. And you can convince voters of lots of things, but you can’t convince them Hillary Clinton is unqualified to be president,” Pfeiffer said.

Pfeiffer and former Obama aides have long said that — contrary to the Clinton campaign’s insistence — the tone of the 2016 campaign has been tame. But Pfeif­fer added that how and when Democrats unify is ultimately up to Sanders.

“The prospect of a Trump or Cruz presidency will be so scary to Democrats that it may heal all wounds,” he added.

The Clinton campaign released two new ads Thursday, both highlighting her efforts to boost small businesses, manufacturing and high-tech jobs in western New York as senator.

Both 30-second spots — one called “Norma” and the other “Every Corner” — focus on her efforts to bring new industry to such cities as Buffalo, Albany, and Rochester.

“Hillary Clinton made a difference in every corner of New York, and that’s what she’ll do in every corner of America,” the narrator says.

On Friday, Clinton is expected to campaign in Buffalo and Rochester — two of the media markets where these ads will air.

Sanders had to put out a separate fire Thursday to avoid alienating another important Democratic constituency. Sanders has drawn criticism for blaming Israel for much higher Palestinian casualties than the country admits during the 2014 conflict in Gaza between Israel and Hamas. But on Thursday afternoon, the Anti-Defamation League, a Jewish civil rights organization, said that it had been in contact with Sanders and he had explained that he had misstated the numbers.

Despite Sanders’s recent streak of caucus and primary wins, Clinton has kept a steady lead in terms of delegates, even discounting the hundreds of superdelegates who have thrown their support behind her. Many question whether it’s even possible for Sanders to win the nomination at this point. And while no one is calling for him to drop out, some are increasingly worried that this seemingly never-ending nominating contest is doing increasing damage to Clinton.

Congressional Democrats, in particular, said they hope this spat serves as a warning to the candidates to tone things down for the good of keeping the party more cohesive than the Republicans.

“We are nowhere near the playground name-calling of the Republican Party,” McCaskill said. “The vitriol and nonsense on the Republican side is a standard that we could never reach.”

“It’s really important that everybody take a pause, that everybody calm down,” Sen. Heidi Heitkamp (D-N.D.) said. “At this point in campaigns, people get tired, say things they don’t mean to, emotions get raw. I think a lot of this will dissipate with a couple good nights’ sleep.”

Gearan reported from Washington. Karoun Demirjian, Mike DeBonis, Paul Kane and David Nakamura in Washington contributed to this report.