Sen. Susan Collins (R-ME) has announced in a Washington Post op-ed that she will not be voting Donald Trump in the general election, saying he "does not reflect historical Republican values," and is "unworthy of being our president." Collins did not signal who she will vote for (indeed she explicitly stated she doesn't support Hillary Clinton), but she stated unequivocally it won’t be the Republican nominee.

"I had hoped that we would see a ‘new’ Donald Trump as a general-election candidate — one who would focus on jobs and the economy, tone down his rhetoric, develop more thoughtful policies and, yes, apologize for ill-tempered rants," Collins laments. "But the unpleasant reality that I have had to accept is that there will be no ‘new’ Donald Trump, just the same candidate who will slash and burn and trample anything and anyone he perceives as being in his way or an easy scapegoat. Regrettably, his essential character appears to be fixed, and he seems incapable of change or growth."

Collins is just the latest Republican to abandon Trump

Collins isn’t the first Senate Republican to jump ship. Illinois Sen. Mark Kirk (R-IL) has said that Trump is "too bigoted and racist" to be president, and run campaign ads attacking him. Sen. Ben Sasse (R-NE) has been saying he can’t support Trump since the primaries. Sen. Lindsey Graham (R-SC) called on fellow Republicans to "un-endorse" Trump back in June. Collins has the most moderate voting record of any Republican senator, so it’s not too surprising she’d join this group.

And this kind of congressional party defection happens during most presidential elections: Recall Democratic-aligned Sen. Joe Lieberman endorsing John McCain in 2008, or Democratic Sen. Zell Miller endorsing George W. Bush in 2004, or then-Republican Sen. Lincoln Chafee writing in George H.W. Bush rather than vote for the younger Bush in 2004.

But Collins’s statement stands out as unusually public and strident, and because it comes amid a slew of similar declarations from prominent Republicans. In the past week, there were announcements Rep. Richard Hanna (R-NY), who said he’d vote for Hillary Clinton in the general election, and Rep. Scott Rigell (R-VA), who is supporting Libertarian nominee Gary Johnson, as well as an open letter signed by senior foreign policy officials from past Republican administrations declaring their belief that Trump is a threat to national security.

In that context, Collins’s statement seems like a sign that the Republican establishment is finally cracking, and prominent elected officials, albeit centrist-leaning ones, are ready to abandon Trump. Collins also stands out because she’s not up for reelection or retiring this year. Hanna is not seeking another term, making it easier for him to abandon Trump without triggering a revolt from his base. Kirk is in a tough battle for reelection in a deep blue state, making it a political imperative that he attack Trump.

Collins’s defection, however, lacks any immediate political motive, and is potentially costly in opening her up to a Trumpian primary challenge when she’s next up for reelection in 2020. It also stands out for its stridency. Collins does not merely criticize Trump — she calls him a threat to national security, expressing a fear that Trump’s "lack of self-restraint and his barrage of ill-informed comments would make an already perilous world even more so."

She says his comments on Judge Gonzalo Curiel "demonstrated a profound lack of respect not only for the judge but also for our constitutional separation of powers, the very foundation of our form of government." She accuses him of "showing complete disregard for common decency."

Those are strong words coming from a senator of Trump’s own party — ones that call to mind Margaret Chase Smith, the Republican who once held Collins’s Senate seat and was among the first to condemn Joe McCarthy in 1950. And Collins’s bluntness, and her willingness to state her refusal to vote for Trump in clear terms, might open the door for other members of the Senate GOP caucus to do the same.