New Hampshire is among key swing states where prominent local Republicans are abandoning their party's standard-bearer in the wake of Trump's string of missteps, erratic statements, and outlandish approach to his White House campaign in the weeks after his nominating convention in July. The trend is also playing out in Ohio and Florida.

The rising stream of Republicans ready to vote for Clinton marks a new phase in the anti-Trump movement that's percolated since he became a serious contender. Whether they are, like Humphrey, grudgingly making deals with their own conscience to vote for Clinton if the race is tight, or endorsing the former first lady, these Republicans are entering territory that would have been unthinkable a year ago: getting behind one of the right's most reviled villains.

The GOP defections are extremely dangerous for Trump, whose campaign is taking on water and listing badly. Winning even a marginal slice of the Republican vote would help Clinton cut off the narrow path Trump has to victory.

The trend shows the rift in the Republican Party is deepening into a chasm, a split driven wider not only by intensifying disgust with Trump but also anger at the party leadership backing him.

"I am disappointed in many of my fellow Republicans. Behind closed doors they rip Trump and roll their eyes and say what an embarrassment he is and how far back he is taking the GOP, but then in public say nothing and just shrug," said Mark Lenzi, a former spokesman for the New Hampshire Republican Party and a staffer on John McCain's 2008 primary campaign, who is backing Clinton. "Because Trump's foreign policy positions especially would be such a disaster for us as a country, I decided I had to speak up and be public about it."

The cochair of Trump's New Hampshire campaign, state Representative Steve Stepanek, dismissed Humphrey as "sort of a kook" who has been anti-Trump from the start, recalling that Humphrey was involved in efforts to block Trump at the Republican National Convention. "He does not represent the mainstream at all in New Hampshire," he said.

Stepanek said pundits continue to underestimate the degree to which Trump has brought new voters into the process.

"We have a very, very strong ground game in New Hampshire," he said.

The campaign is working closely with the state Republican Party and the Republican National Committee, he said, and plans to open more offices than Romney had in 2012.

"I think it's going to be a very competitive race," he said.

To be sure, many elected leaders in New Hampshire continue to support Trump, including Senator Kelly Ayotte, who is locked in a tough election battle of her own. And Trump did win 35 percent of the Republican vote in the primary — 20 points more than his nearest competitor.

But the state, with a history of moderate Republicanism and ticket-splitting voters, is particularly fertile ground for rebellion against the top of the ticket. Sure, New Hampshire voters helped launch Trump by delivering him a resounding primary victory after his loss in Iowa. But they also gave moderate Kasich a solid second-place finish.

A post-convention WBUR poll of likely voters in New Hampshire showed Clinton leading Trump by 15 points in the state, a turnaround from three months earlier when the poll showed Trump up slightly.

Among registered Republicans, 63 percent said they are voting for Trump, while 14 percent said they planned to vote for Clinton. Only 4 percent of registered Democrats said they planned to back Trump.

Clinton also has the advantage among the state's robust independent faction, leading Trump among them 49 percent to 23 percent.

Clinton forces are striving to capitalize, but even without their prodding, well-connected GOP figures in New Hampshire and beyond are pitching fellow Republicans and independents to join the former secretary of state's cause. Powerhouse GOP donors are now raising money for Clinton. Some rank-and-file Republicans are even volunteering for unglamorous phone-banking and door-knocking jobs.

"This is really unusual to have so many prominent leaders within the Republican Party, so many intellectuals within the Republican Party, and so many former Republican officials both breaking with the presidential nominee and endorsing the opponent," said Allan Lichtman, a political historian at American University.

There have been, of course, times Republicans have been dissatisfied with their nominee. The prime example: the 1964 campaign of Barry Goldwater, who was too far to the right for many mainstream GOPers, Lichtman said. But they did not openly break with Goldwater, or endorse the Democratic incumbent, President Lyndon B. Johnson, in the way prominent Republican figures are doing now, he said.

"We really haven't seen this kind of earthquake ever," Lichtman said.

The Clinton campaign's active wooing of Republicans was on full display at the Democratic National Convention in Philadelphia. Two Republicans who decided to vote for Clinton got prime speaking slots. Former New York mayor and self-made billionaire Michael Bloomberg, an independent, took the stage to endorse Clinton, too.

The Clinton campaign has launched "Together for America," with a starter batch of nearly 50 Republicans and independents from government, the military, and business who are endorsing Clinton. In the weeks ahead some of these newly minted Clinton backers will serve as campaign surrogates in the media and in battleground states.

The repudiation of Trump and turn toward Clinton by party figures in these battlegrounds could persuade rank-and-file voters to do the same.

"It gives them a little courage to move in that direction," said Fergus Cullen, a former New Hampshire GOP chairman. Cullen counts himself in the "Never Trump" camp but said he will not be voting for Clinton. However, he predicted about a quarter to a third of the New Hampshire GOP vote could swing to Clinton.

GOP insiders say Humphrey's anti-Trump activism is noteworthy given his long record as a rock-ribbed conservative who doggedly pursued fights against abortion and congressional pay raises during his two terms in the Senate from 1979 to 1990.

Humphrey is a leading signatory of a letter being sent to RNC chairman Reince Priebus next week pressing him to cut off money and other resources to Trump and focus on shoring up vulnerable GOP seats in the House and Senate.

Humphrey said he feels it's incumbent upon him to speak out against Trump.

"Presumably I have a little bit of standing as a former Republican senator, conservative Republican at that, such that I feel obligated to have whatever effect I can have," he said.

For Republican Kelleigh Murphy, chairwoman of the Bedford town council, her anti-Trump vote will come down to cold, hard math. If Clinton is up five points or more over Trump the week before the election, she will vote for Libertarian Gary Johnson. Tighter than that or Clinton down? She will vote for the Democrat.

"Trump is a loose cannon, and if I need to employ a mathematical formula to attempt to prevent that loose cannon from mucking up my country, you can bet that is what I am going to do," she wrote on her Facebook page.

Lenzi, the former New Hampshire Republican Party spokesman, on the other hand is all in, even though it will be the first time he has supported, and worked for, a Democrat in his life, going back to working on former senator Richard Lugar's 1996 presidential campaign.

Beyond going public with his support for Clinton, Lenzi said he is helping the Clinton campaign with voter-targeting and get-out-the-vote efforts in New Hampshire. He also is assisting the national campaign on foreign policy issues and outreach to certain ethnic groups including Polish-Americans (a former Peace Corps volunteer in Poland, he speaks the language) in swing states.

Betty Tamposi, a former assistant secretary of state in the George H.W. Bush administration with longstanding ties to state GOP circles, also publicly embraced Clinton. Now she's talking to any of her Republican friends in New Hampshire who will listen, seeking to persuade them to join her on the Clinton bandwagon. She is in touch with the Clinton campaign, too, "sharing information," she said.

"In New Hampshire, we are a word of mouth state. People in New Hampshire still talk to each other," said Tamposi, who is also a former state legislator. "I do think it will make a difference."

Victoria McGrane can be reached at victoria.mcgrane@globe.com. Follow her on Twitter @vgmac.