Donald Trump is firing back as a growing number of Republican officials come out against his candidacy, describing them as the kind of “Washington insiders” he’s running against – as these officials raise serious concerns about his character and fitness to lead.

The most recent defections involved a group of 50 former security and foreign policy officials from GOP administrations dating back to Richard Nixon.

This comes on top of:

Maine Republican Sen. Susan Collins declaring in an op-ed she cannot support Trump

An anti-Trump independent candidate entering the 2016 race

Conservatives reportedly circulating a petition calling on the Republican National Committee to convene a meeting where Trump could potentially be replaced

The letter from ex-security officials claimed Trump would be a “dangerous” president who would put the nation at risk.

“None of us will vote for Donald Trump,” they wrote. “… Most fundamentally, Mr. Trump lacks the character, values, and experience to be President.”

But Trump, in a series of interviews and statements, dismissed the letter as the work of “insiders.”

“I got 14 million votes … Which is the most in the history of the primary system [for] Republicans. Frankly, people are fighting exactly what these people are doing,” he told Fox Business Network on Tuesday.

“The names on this letter are the ones the American people should look to for answers on why the world is a mess, and we thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place,” he said in a written statement. “They are nothing more than the failed Washington elite looking to hold onto their power, and it’s time they are held accountable for their actions.”

The co-signers of the letter include officials who served in Republicans administrations dating back to the Richard Nixon administration. It also includes officials who served more recently, under George W. Bush, like ex-Homeland Security Secretary Michael Chertoff.

Another co-signer, Roger Zakheim, former deputy assistant secretary of defense under Bush, told Fox News on Tuesday he’s hoping to have “another choice” by November.

Asked if he’d vote for Hillary Clinton, he responded, “I don’t have to choose between bad or worse. I’m not going to vote for either.”

Meanwhile, Collins published a scathing op-ed in The Washington Post declaring Trump is “unworthy of being our president.”

She wrote: “My conclusion about Mr. Trump’s unsuitability for office is based on his disregard for the precept of treating others with respect, an idea that should transcend politics. Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities.”

Amid the intra-party turbulence, an anti-Trump candidate threw his hat in the ring Monday. Evan McMullin, former CIA officer and former chief policy director with the House Republican Conference, is running as an independent, though he doesn’t have high name recognition and has missed many state ballot deadlines.

The petition calling for a special meeting to replace Trump also faces a tough climb. According to the Associated Press, Trump critics are looking at party rules that allow for the RNC to replace a nominee in the case of "death, declination, or otherwise" – if they get signatures from 16 RNC members from as many states.

Meanwhile, Republican National Committee chief strategist Sean Spicer fired off a memo Tuesday morning reminding “interested parties” of the risks of a Hillary Clinton presidency.

“The American people have had enough of failed status quo policies which have left them less hopeful in our country’s future. They have had enough of serially dishonest, corrupt, and self-interested career politicians,” he wrote. “If we do not elect Donald Trump president, these will be the inevitable, unaffordable hallmarks of a Clinton presidency.”