NEW YORK — The contest between Hillary Clinton and Sen. Bernie Sanders grew increasingly nasty 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 a 10-day sprint here ahead of the April 19 primary, the strain and resentment of a hard-fought and unexpectedly long contest boiled over repeatedly in interviews, speeches and other appearances. Sanders 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 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.”

But 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 last 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.

Other Democrats are worried about potentially longer-term fallout for the party of an increasingly personal conflict between Sanders and Clinton. Most of these 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 Thursday, White House spokesman Eric Schultz said Obama believes Clinton “comes to the race with more experience than any non-vice president” in recent campaign history.

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 support of what he termed a series of “disastrous” trade deals.

“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-Missouri.

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

Some Democrats said they found Sanders’ words 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-Virginia, 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 Barbara Lawton, a former Wisconsin lieutenant governor and Sanders supporter. “I think the 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 told 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 quiz to bolster the argument that her resumé and practical experience make her qualified.

Trying to convince Democrats that Sanders is more of a typical politician than a principled crusader has been an ongoing – and largely unsuccessful – effort for Clinton. But the latest skirmish gave her a new chance to do so.

This time, Sanders may have given them 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.