By David Jackson and Eliza Collins

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.

READ MORE: NRA launch $3m anti-Hillary Clinton ad buy taking aim at gun rights

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 for The Washington Post.

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.”

READ MORE: NRA launch $3m anti-Hillary Clinton ad buy taking aim at gun rights

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.

READ MORE: NRA launch $3m anti-Hillary Clinton ad buy taking aim at gun rights

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.