On Wednesday, something happened that has never, to this point, happened in the Republican nomination contest: A member of Congress endorsed Donald J. Trump.

Actually, two members of Congress — Rep. Duncan Hunter of California and Rep. Chris Collins of New York — announced they were backing the billionaire.

"Donald Trump has clearly demonstrated that he has both the guts and the fortitude to return our nation’s jobs stolen by China, take on our enemies like ISIS, Iran, North Korea and Russia, and most importantly, re-establish the opportunity for our children and grandchildren to attain the American Dream," said Collins, in a statement given to the Buffalo News. "That is why I am proud to endorse him as the next president of the United States."

"We don't need a policy wonk as president. We need a leader as president," Hunter told Politico's Nick Gass. And he hinted there may be more endorsements to come in the Capitol: "I think you have more Trump supporters in Congress. They just have to come out of the closet, so to speak."

Trump still has far fewer endorsements than Marco Rubio, Ted Cruz, and even John Kasich, as you can see in FiveThirtyEight's tracker. But these two endorsements are still a milestone in the race, reflecting that a growing number of political elites are concluding that the billionaire will likely be the GOP nominee — and that some of them are now eager to get behind the Trump parade.

Both of Trump's new endorsers are from blue states — and that's bad news for Rubio

The most worrying sign for GOP elites is that these endorsements mean the establishment's last, desperate plan to stop Trump may be slipping away. Both Collins and Hunter are from blue states (they represent suburbs of Buffalo and San Diego County, respectively).

That's not to say their voters are liberal — both districts are quite conservative. But they're blue-state Republican primary voters — and the latest theory that Rubio can beat Trump head to head once the field winnows basically requires Rubio, not Trump, to win blue-state primaries.

That's because the calendar has more delegate-rich blue states set to vote after March 15, when the field likely will winnow to Trump and Rubio. About 60 percent of the remaining primary and caucus delegates will be from either Northeastern states or California.

So if Trump does build up a big delegate lead in early March — which he will do if recent polls are accurate — Rubio will need to rack up big margins in many of those states to catch up to him. But Trump has done well in the Northeast so far, and is polling well in states like Massachusetts and Vermont.

Hunter's endorsement of Trump could be even more worrying if the contest stretches on for several more months. California votes dead last, on June 7. But 172 delegates will be up for grabs there. And Rubio can't count on Latino anti-Trump turnout to help him, since there's a lot of regional variation in California and most of its delegates will be awarded to the winners of each of its 53 congressional districts — many of which are filled with white rather than Latino Republican primary voters.

All this goes to show, again, that unless the key dynamics of the race change very quickly, Republican elites are running out of time to stop Trump.