David Jackson, and Eliza Collins

USA TODAY

Donald Trump is facing a rising and unique group of outspoken opponents.

They're called Republicans.

Maine Sen. Susan Collins has joined a growing group of anti-Trump Republicans that now includes other lawmakers, national security officials, GOP donors, and various party professionals, including a former House staff member who says he is mounting his own "Never Trump" presidential bid.

The GOP presidential nominee "does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country," Collins said in an op-ed forThe Washington Post.

Republican Sen. Susan Collins says she can't support Trump

While the rising number of party defections is unique, Trump says he is fighting the entrenched political establishment that has failed the United States.

"I am running against the Washington insiders, just like I did in the Republican Primaries," Trump tweeted Tuesday. "These are the people that have made U.S. a mess!"

Hillary Clinton's campaign is pushing the idea of "Republicans for Clinton" and highlighting announcements by anti-Trump GOP members.

"A growing number of Republicans are deciding that this election can’t be about party — it’s about doing what’s right for the country and electing someone who actually has the qualifications, fitness and temperament to serve as President and Commander-in-Chief,” Clinton spokesman Jesse Ferguson told USA TODAY.

On Tuesday, GOP donor Harry Sloan officially endorsed Clinton. Sloan, a former CEO of MGM, worked for previous Republican presidential nominees John McCain and Mitt Romney, and fundraised for Ohio Gov. John Kasich during this year's Republican primaries.

In an interview with USA TODAY, Sloan said Clinton's focus on energy and immigration aligned with his priorities and he was impressed with her on a variety of other topics. He also said that her focus on infrastructure and education spending could help her with business Republicans.

“I want to reach out to Republican leaders who held positions like I did on the 2016 campaigns, like I did with Kasich ... and bring them over," he said.

Clinton visited a South Florida health center with a Republican on Tuesday, Miami-Dade County Mayor Carlos Gimenez. While Gimenez — who backed Jeb Bush and then Marco Rubio in the primaries — has not endorsed Clinton, her press operation fired off a story from the MiamiHerald announcing that the two would appear together.

And Cindy Guerra, a former Republican chair of Broward County in Florida, also backed Clinton Tuesday, telling the Miami Herald that “it’s a matter of country over party — as cheesy and goofy as that sounds.”

Later Tuesday a group of former Republican officials announced they'd be backing Clinton too. The group, R4C16 (Republicans for Clinton '16), included more than a dozen people.

James K. Glassman, who was under secretary of State for Public Diplomacy and Public Affairs in the George W. Bush Administration, said in a statement that a vote for Clinton was a vote for Republicans down ballot.

“In voting for Secretary Clinton in this election, we will also be voting for Republicans in Senate and House races. Retaining the Congress is critical for those of us who, unlike the man the GOP nominated, continue to believe in the principles of the party of Lincoln and Reagan – liberty and respect for the individual," he said.

Many of the "Never Trumps" are older Republicans who have seen the party turn more conservative in recent decades. That group ranges from Brent Scowcroft, a national security adviser to presidents Gerald Ford and George H.W. Bush, to William Ruckelshaus, who headed the Environmental Protection Agency for presidents Richard Nixon and Ronald Reagan.

Other Trump opponents backed other candidates in the bruising Republican primaries. Rivals Ted Cruz and Kasich have pointedly refused to endorse the GOP nominee.

One GOP lawmaker, Rep. Scott Rigell of Virginia, has endorsed the Libertarian candidate, former New Mexico governor Gary Johnson.

Meanwhile, a new independent candidate, Evan McMullin — a former CIA operative and chief policy director for House Republicans — said in an open letter that while Clinton "is a corrupt career politician who has recklessly handled classified information," Trump is really no better.

Little-known GOP congressional aide to challenge Trump for presidency

"Given his obvious personal instability, putting him in command of our military and nuclear arsenal would be deeply irresponsible," McMullin said.

In her op-ed, Collins echoed other Republican critics in citing Trump's behavior, including his mocking of a reporter with a physical disability, his attacks on a federal judge's "Mexican heritage" and his dismissal of a Muslim couple who lost a son in Iraq.

Collins' announcement came shortly after 50 national security officials signed a letter citing Trump's questioning of military alliances, as well as his "erratic" behavior.

"He would be the most reckless president in American history," the letter said.

Asked about that letter on Fox Business Network, Trump said some of his critics "would have loved" to have been part of his campaign, but he didn't want them.

50 GOP officials: Trump would be 'most reckless President in American history'

Previous elections have also seen party defections.

During the 1980s, President Ronald Reagan attracted the support of many Democrats, especially in the South, who were en route to becoming Republicans as part of a larger political realignment across the country. Once solidly Democratic, the South is now Republican territory.

Barry Goldwater, who brought a stronger conservative ideology to the Republican Party when he won its presidential nomination in 1964, also faced many critics inside the party. But political analyst Stuart Rothenberg pointed out that many of those critics in 1964 wound up endorsing — or at least not actively opposing — Goldwater, and that has not been the case this year with Trump.

"This is off the charts," he said.

The Republican opposition comes at a time when Trump is trying to build a coalition and address problems with large groups of voters, such as women and Hispanics.

GOP pollster Whit Ayres said recent GOP presidential election winners received at least 91% of the Republican vote —Trump is now in the upper 70s.

"He's about 10 to 15 points from where he needs to be among Republicans," said Ayres, who worked for Rubio during the primaries.

Jennifer Duffy, a senior editor with The Cook Political Report, said Republican opposition "doesn't help" Trump, but it's hard to assess the impact right now because polls are volatile in the wake of the recent party conventions.

"We don't know yet," she said.

Trump and his aides said his emphasis on trade and lost manufacturing jobs is helping him make inroads with blue-collar voters in states like Pennsylvania and Ohio. They also said many voters across the country resent the carping from the Republican "elite."

In his Fox interview, Trump said he doesn't plan to change the approach that got him this far.

"I certainly don't think it's appropriate to start changing all of the sudden when you've been winning," Trump said. "I mean, I've beat many people and now we're down to one. And we'll see how it all works out. But I think it's going to work out well."