When we talk about impeachment, it’s common to note that at the moment it seems virtually impossible that 20 Senate Republicans would vote to convict President Trump, which would be required to remove him if every Democrat voted for removal. That’s perfectly true, and reasonable to keep in mind.

But even if Trump survives impeachment, Republican defections could still create an enormous problem for him, one that would threaten his chances of being reelected.

Cracks in his unified Republican support are already appearing. And for Trump to get reelected, he needs that support to be almost unanimous. His strategy depends on it.

Sen. Lindsey O. Graham (R-S.C.), lickspittle par excellence, has introduced a resolution condemning the House impeachment inquiry as illegitimate. It’s comically insincere playacting. Graham knows full well there’s nothing illegitimate about the inquiry; whether you think Trump should be impeached or not, it’s fully within the House’s power, and Democrats are going about it in a perfectly methodical way. It’s the Republicans who are acting like clowns.

Whatever he thought he would accomplish with his resolution, Graham set up a loyalty test for the 53 Republican senators to send a signal to the president himself and to their voters: Are you willing to stand behind Trump no matter what?

What he got was not a unanimous “Yes!” While 46 Republicans signed on, seven refused: Susan Collins, Cory Gardner, Lisa Murkowski, Mitt Romney, Lamar Alexander, Johnny Isakson, and Mike Enzi. It’s a revealing group.

Enzi, Isakson, and Alexander are all retiring. Murksowski has cultivated a reputation for moderation and principle, one in which opposing Trump is key to her brand (she voted against repealing the Affordable Care Act and opposed Brett M. Kavanaugh’s confirmation to the Supreme Court). Mitt Romney has become Trump’s chief Republican critic in Congress.

And Susan Collins and Cory Gardner? They’re the two most vulnerable Republicans up for reelection in 2020. And the only two representing states Hillary Clinton won in 2016.

To say they’re in a tough spot would be an understatement. In order to get reelected, they need to both hold on to their Republican voters and win over some independents and Democrats. If they turn against Trump, they alienate GOP loyalists, but if they support him too slavishly, they risk alienating everyone else. They might be able to duck the question for now, but come the Senate trial it will no longer be possible.

This is where you object, “But most Republicans will never abandon Trump!” Which is true. Even most of those appalled by his misdeeds know that their constituents will stick with him no matter what is revealed. There’s no margin for them in opposing him, because they represent heavily Republican districts and states.

But you don’t need a large number of Republicans to oppose Trump to have a profound impact on the 2020 election. All you need is some kind of critical mass, enough to signal to moderate Republican voters that you can still be a Republican and vote for a Democrat in 2020, or vote third party, or not vote at all.

Imagine, then, the vote that occurs after the Senate trial. By then we’ll surely know even more about all the impeachable things Trump did. All 47 Democrats vote for removal. So do Collins, Gardner and Romney. That makes 50. Throw in just one more — Lisa Murkowski or Lamar Alexander, or maybe another vulnerable senator like Martha McSally of Arizona — and you have a majority of the Senate voting to remove Trump, even if it’s far short of the 67 votes needed.

If that occurs, “Republicans Divided!” would become one of the big stories coming out of impeachment. As 2020 progresses, candidates like Gardner and Collins could decide, if they seem to be headed for defeat, that the only thing that will save them is full-throated opposition to Trump. The same could be true of other Republicans running for the House or governorships.

The result would be a good-sized collection of Republican candidates opposing Trump all over the country — enough to signal Republicans everywhere that defection from this president is not treason against their party.

And that would almost guarantee Trump’s defeat.

Trump believes that he got elected not by reaching across the middle but by exciting his base, so everything he says and does is meant to cultivate that base. That’s his 2020 strategy as well. For it to work, however, he needs not only to get his supporters to the polls but to keep moderate Republicans from defecting or staying home. The entire election must be seen to be as partisan as possible, so Republicans say what they did in 2016: I may not like him, but as a Republican I’m going to support him.

The flip side of Trump’s strategy of hate and fear is that it makes it impossible for any Democrats to vote for him. So if he suffers a significant number of defections among Republican voters, he’s doomed.

At this point nothing is certain; it could well be that by Election Day Trump’s support among Republican voters is as firm as it was in 2016. But he can ill afford to lose any Republican support. And it might already be happening.

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