Opinion writer

Support for President Trump among congressional Republicans has been all but absolute over the past two years. But there’s a drama playing out right now over his declaration of a national emergency in order to divert funds to border wall construction that portends a more complicated 2020 for both the president and his allies.

If rifts are developing in that relationship, it could lead to a spiral in which members of Congress see it as in their interests to separate themselves from the president, and in response he lashes out at them, which only encourages them to distance themselves from him even more. What has been a tight alliance could break down quickly.

Let's begin with the latest on the emergency declaration:

President Trump issued a fresh veto threat Thursday morning after three Republican lawmakers showed up at the White House the previous night, seeking a way to stave off an expected rebuke in the Senate over his national emergency declaration at the U.S.-Mexico border.

Sens. Ted Cruz (Tex.), Lindsey O. Graham (S.C.) and Ben Sasse (Neb.) unexpectedly arrived as Trump was having dinner in the residence with family, according to two people familiar with the episode, who requested anonymity to discuss a sensitive matter.

The senators argued for a last-minute proposal aimed at satisfying concerns of senators who are inclined to vote Thursday afternoon for a measure nullifying Trump’s declaration, the people said. Details on their proposal could not immediately be learned.

The people familiar with what transpired said a White House lawyer was brought in to explain why the proposal wouldn’t work, and the discussion was ultimately unsuccessful, irritating Trump, who renewed his veto threat with an early morning tweet on Thursday.

This came after Trump rejected a separate proposal by Republican Sen. Mike Lee (R-Utah) that would have required that emergency declarations expire without action by Congress to approve them. Trump is simply not going to accept any limits on his authority.

While there’s probably a part of the president that likes the idea of standing alone against all the weak-kneed forces of Washington unwilling to protect America from an invasion of murdering, rapist drug dealers, there’s surely another part of him that’s enraged by the idea that any Republican would defy him. According to The Post’s Josh Dawsey, White House officials think 10 to 12 Republicans will join with Democrats in the vote to roll back the emergency declaration, a high-enough number to suggest that party unity behind Trump is cracking.

Not only that, seven Senate Republicans just joined with Democrats in a vote to end U.S. support for the Saudi-led war in Yemen, another rebuke to Trump administration policy.

And in an action that may make Trump even more angry, the House just voted 420-to-0 to urge Attorney General William P. Barr to publicly release Robert S. Mueller III’s report on the Russia scandal in its entirety once it is complete.

We should be clear that, for most of the potential defectors on the emergency declaration, this is a vote on principle. They believe the president’s powers are limited and he shouldn’t be able to circumvent Congress with a phony emergency declaration whenever he doesn’t get what he wants. But of course, politics is never far from anyone’s mind.

Now let’s think about what might happen in the second half of this year and 2020 as the presidential campaign really gets going. Although the race will probably be close, there’s a good chance Trump will be running behind in the polls for much of it; his approval never moves much from around 40 percent, and if the economy turns down, as many believe it will, he’ll be in real trouble.

So every Republican running for the House and Senate will have to ask themselves: How closely do I want to tie myself to Trump if he might be going down? If they’re running in a strongly Republican district or state, the question is easy to answer. But there are multiple senators up for reelection in 2020 who could face tough races. I count at least six: Martha McSally (Arizona), Cory Gardner (Colorado), David Perdue (Georgia), Joni Ernst (Iowa), Susan Collins (Maine), and Thom Tillis (North Carolina).

The worse the president is doing in the polls, the more incentive they’ll have to display their independence to home-state voters by disagreeing with him (and that’s not to mention others who may do so for substantive reasons from time to time). And the more they do, the angrier Trump will get, and the more likely he’ll be to start campaigning not just against Democrats but against Congress in general, including members of his own party.

It’s not hard to imagine Trump, who is absurdly sensitive to personal slights, doing a rally in a place such as North Carolina and pouring derision on Tillis for some recent vote, then telling the crowd that it doesn’t matter who else they vote for as long as they return him to the Oval Office. Which in turn could encourage the senator he’s insulting to distance himself further from the president.

From Trump’s perspective, all that matters is that he be reelected. Republicans don’t have any remaining significant legislative goals, and he himself certainly doesn’t. He needs the Senate to stay Republican in order to confirm his appointments, but would that be enough to overcome his natural instinct to make an enemy out of anyone who defies him? I doubt it. All of which suggests that the GOP could be in for a bumpy 2020.