Yesterday, a slew of high-profile Republicans announced they would not vote for Donald Trump, with many saying they would vote for Hillary Clinton.

Almost to a man, the defectors are remnants of the George W. Bush administration. 

First, Senator Susan Collins became the least surprising defector, saying she can't support Trump for president.

The Hill:

"I will not be voting for Donald Trump for president,” she declared. 

“This is not a decision I make lightly, for I am a lifelong Republican. But Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country.”

In the piece, Collins compares the GOP nominee to Sen. Bernie Sanders (I-Vt.), who captured attention and grew support for his challenge to Democratic nominee Hillary Clinton.

She said it “ became apparent that, much like” Sanders, Trump “was connecting with many Americans who felt that their voices were not being heard in Washington and who were tired of political correctness. 

“But rejecting the conventions of political correctness is different from showing complete disregard for common decency,” she wrote.

Instead of just “shedding the stilted campaign dialogue,” she wrote, Trump “opted for a constant stream of denigrating comments, including demeaning Sen. John McCain’s (R-Ariz.) heroic military service and repeatedly insulting Fox News host Megyn Kelly.”

Collins has been a vocal Trump critic and hasn’t been shy about rebuking the real estate mogul.

Also yesterday, 50 GOP national security experts signed a letter declaring they could not vote for Trump.

The letter, signed by aides and Cabinet members of past GOP administrations including George W. Bush's and Richard Nixon's, declared that none of the officials will vote for the GOP nominee because they “are convinced that he would be a dangerous president and would put at risk our country’s national security and well-being.

“Mr. Trump lacks the character, values, and experience to be President,” the letter says. “He weakens U.S. moral authority as the leader of the free world.”

“He appears to lack basic knowledge about the belief in the U.S. Constitution, U.S. laws, and U.S. institutions, including religious tolerance, freedom of the press, and an independent judiciary.”

The officials predict that Trump "would be the most reckless president in American history."

Some of the prominent officials include Michael Hayden, former director of both the CIA and National Security Agency; Michael Chertoff, former Secretary of Homeland Security for Bush and President Obama; John Negroponte, former director of national intelligence under Bush; Tom Ridge, former homeland security director under Bush as well as former governor of the battleground state of Pennsylvania; as well as others who have served as trade representatives, national security advisers and ambassadors.

And, not surprisingly, two former GOP EPA chiefs refuse to back the nominee and have endorsed Clinton:

Two former Environmental Protection Agency (EPA) chiefs who served under Republican presidents are backing Democrat Hillary Clinton in the November election

William Ruckelshaus, who served under presidents Richard Nixon and Ronald Reagan; and William Reilly from the George H.W. Bush administration said Republicans should be outraged by Donald Trump’s threats to undo major environmental policies.

“Donald Trump has shown a profound ignorance of science and of the public health issues embodied in our environmental laws,” the men wrote in a letter released Tuesday morning by the Clinton campaign.

“He hasn’t a clue about Republicans’ historic contributions to science-driven environmental policy: the incontestable environmental improvements in the restored lakes and rivers, the acid rain controls, the reduction of key air pollutants by 70, 80 and 90 percent, and the sharp decline in pollution from automobiles even as their numbers more than doubled.”

Ruckelshaus and Reilly add to the growing list of Republican officials, including former federal officials under GOP presidents, denouncing Trump and endorsing Clinton.

In the two days, Clinton’s secured endorsements from former George W. Bush aide Lezlee Westine, Reagan political director Frank Lavin and ex-Michigan Gov. William Milliken.

Similarly to many of their fellow Republicans, Ruckelshaus and Reilly framed a Trump administration as dangerous to the causes for which they feel strongly.

What is happening is what Glenn Reynolds refers to as a "preference cascade," sparked by polls showing Trump falling behind by double digits nationally and by significant margins in states like Georgia and Michigan. 

Perhaps most worrying is that Clinton has the support of 92% of people who identify as Democrats, while Trump can manage only 79% support from Republicans, according to a Monmouth poll.  Trump may not be down 13%, as the Monmouth poll shows, but unless he can find a way to bring many of those Republicans back, it is going to be hard to remain competitive with Clinton.

A lot of these Republicans probably wouldn't have supported Ted Cruz, either, so the onus is not all on Trump.  But the candidate can ill afford to bleed any more support, even from moderate Republicans.