Former vice president and Democratic presidential candidate Joe Biden speaks during a campaign event Tuesday in Berlin, N.H. (Elise Amendola/AP)

Most of the Democratic presidential field blistered Joe Biden with criticism Wednesday for his position on abortion rights, isolating the candidate at the top of the polls on a topic that has animated the Democratic base.

Biden’s stance that he supports a law sharply limiting the use of federal funds for abortions created one of the clearest indications yet of a growing division among Democrats and a feistiness in the field, which on Wednesday displayed newfound willingness to criticize the front-runner.

Recent days have laid bare potential vulnerabilities for Biden, whose greatest strength has been the perception that he is the most electable candidate, and the extent to which he is testing the leftward Democratic Party thrust that is embraced by many of his opponents.

Biden has been attacked by fellow Democrats for his positions on abortion, trade and climate change. His campaign acknowledged that sentences lifted from other sources were used in his policy papers, an echo of the plagiarism accusations that drove him out of the 1988 presidential race.

Worries are percolating outside his campaign that he is not doing enough to organize in the early-voting states.

“Maybe he’s not worried about Iowa,” said Kelcey Brackett, chairman of the Muscatine County Democrats. “But, at this point, he might want to pick up the pace.”

Since Biden entered the race, he has demonstrated a strength that has surprised many Democrats, fueled his fundraising and helped keep him at the top of the polls. But Biden’s steady standing has inspired competitors to push back, opening up lines of attack that they hope will cause voters to reassess him.

Those perceived openings for attack include his past support of free trade, his foreign policy stances and the relative moderation of his positions on health care and the environment — instances in which Biden breaks with key constituencies of the party he seeks to lead.

That was highlighted on Wednesday when his campaign issued a statement confirming that unlike much of the rest of the field, Biden is not calling for a repeal of the Hyde Amendment, which restricts the use of federal money to pay for abortions.

Because of the amendment — which dates to 1976, when Biden was a young senator — poorer Americans reliant on Medicaid do not generally have access to abortion. The amendment allows for exceptions only in the case of rape, incest or to save the mother’s life.

Biden would back repealing the amendment “if abortion avenues currently protected under Roe were threatened,” his campaign told NBC News, referring to the 1973 Supreme Court decision in Roe v. Wade, which legalized abortion nationwide.

That put him out of step not only with competitors and key activist groups, but also with the platform the party adopted in 2016. During the Democratic National Convention that year, the party included a plank calling for the repeal of the Hyde Amendment, describing it as among the “federal and state laws and policies that impede a woman’s access to abortion.”

“The Democratic Party platform is crystal clear in supporting the right to safe, legal abortion and repealing the Hyde Amendment, a position held by the majority of voters,” Kelley Robinson, executive director of the Planned Parenthood Action Fund, said in a statement Wednesday taking aim at Biden’s position. “Supporting Hyde isn’t good policy or politics.”

In a separate statement, Ilyse Hogue, president of the abortion rights group NARAL Pro Choice America, said there is “no political or ideological excuse” for Biden’s support for the amendment.

“His position further endangers women and families already facing enormous hurdles and creates two classes of rights for people in this country, which is inherently undemocratic,” Hogue said. “Differentiating himself from the field this way will not earn Joe Biden any political points and will bring harm to women who are already most vulnerable.”

Stephanie Schriock, the president of Emily’s List, a political action committee that works to elect Democratic women who support abortion rights, called it “unacceptable” for a major Democratic White House contender to support the Hyde Amendment.

Biden’s campaign did not respond to a request for comment, but some of his advisers shrugged off the controversy as one that has resonance among Washington-based groups and elites but not something that will change the candidate’s standing among average voters.

Biden has built his political brand as someone willing to make political compromises.

In his 2007 book, “Promises to Keep,” Biden wrote proudly of having sustained “my middle-of-the-road position on abortion for more than 30 years.”

“I still vote against partial birth abortion and federal funding, and I’d like to make it easier for scared young mothers to choose not to have an abortion,” he wrote, “but I will also vote against a constitutional amendment that strips a woman of her right to make her own choice.”

A 2016 Harvard Public Health/Politico poll showed that 58 percent of Americans opposed changing the current policy to allow Medicaid funds to be used to pay for abortions. But the challenge for Biden is that among likely Democratic voters, 55 percent favored changing the law, while 37 percent opposed it.

Biden donors are not concerned about the recent criticisms of him, including over abortion and the missing attribution in his policy statements, said Jon Cooper, a longtime Democratic fundraiser and Biden supporter.

“It’s not even a blip on their radar screen,” Cooper said. “With the caveat that it’s way early and politics can change in a nanosecond, they like the way the campaign is coming together over a relatively short period of time.”

Cooper said the donors he has talked to recently are pleased to see Biden roll out his policy proposals, which he said are showing the candidate can appeal to independents and moderate Democrats.

“They like that his lead in the polls seems to be solidifying. It’s an enviable position to be in,” ­Cooper said.

Regarding the recent criticisms, Cooper said Biden has shown that he would evolve on policy matters “not just because the political winds are blowing in a direction, but it’s the right thing to do.”

“I don’t want to downplay anything; obviously these are important issues and valid for organizations and perhaps other candidates to bring up,” Cooper said, adding that Biden has shown his ideas and stances are “all from the heart. That’s who Joe Biden is, always has been.”

One longtime Democratic fundraiser supporting Biden, speaking on the condition of anonymity to discuss the criticisms frankly, said the critiques from the left over some of his stances are overblown. He said that he has not heard concerns from donors and that the loudest recent shots at Biden have come from a small group of primary voters who are the most vocal online.

“There is zero concern because real Democratic primary voters are not Twitter Democratic voters,” he said. “As long as Joe stays laser-focused on the middle class, and his bona fides on progressive issues in general, this is nothing that is going to knock him off the block. He’s been an eight-year vice president to Barack Obama.”

He added that the more Biden is defined by critics as moderate, the more he will stand out to voters as a candidate who can unify the party against President Trump in 2020.

Yet several party leaders in first-voting Iowa indicated that they were irked about not having interacted much, if at all, with Biden’s campaign and have been frustrated that he is one of the few candidates skipping a state party fundraising dinner this weekend in Cedar Rapids.

They detect little work on the ground regarding organizing and hiring field staffers in the state, which was one of the problems Biden had during his two previous presidential campaigns. One longtime party leader said she attended a recent Biden event in eastern Iowa and was shocked to see that there was no sign-in table for attendees and that the campaign was not trying to keep in touch with those who showed up.

Another activist in western Iowa said that while staffers for several campaigns have been organizing community gatherings and have been meeting with local leaders and recruiting volunteers, she has not seen Biden’s campaign do the same.

Felicia Sonmez contributed to this report