I think he is more vulnerable than many people realize.

First, there are the political risks that his current standing creates for other Republicans. It’s true that his approval rating has been notably stable, around 40 percent. It’s also notably weak. Thus the Republican whupping in the midterms.

Second, Trump’s political fortunes are more likely to deteriorate than improve this year. The economy isn’t likely to get a lot stronger. The various investigations aren’t going away. And Trump will surely commit more unforced errors, like the government shutdown. “It’s still difficult to predict how all this ends,” the political scientist Jonathan Bernstein wrote on Friday, about the shutdown. “But it’s hard to see it ending well for Republicans.”

Third, Republican support for Trump may remain broad, but it’s shallow. Trump has already faced far more intra-party criticism than most presidents. Since the midterms, it seems to be growing. Jim Mattis, the defense secretary, resigned and criticized Trump while doing so. Mitt Romney entered the Senate by once again turning against Trump. Collins and Gardner have started grumbling about the shutdown.

As Republicans begin looking nervously to 2020, their willingness to break with Trump may increase. For some of them, their political survival may depend on breaking with him. If that happens, it’s quite possible that his approval rating will begin to drift below 40 percent — and the bad news will then feed on itself.

No, none of this is guaranteed. Democrats could overreach, by quickly impeaching Trump and thereby uniting Republicans. Or Trump could end up navigating the next few months surprisingly well. But that’s not the mostly likely scenario.

The normal rules of politics really do apply to Trump. He won a shocking victory in 2016, and his opponents have lacked confidence ever since. They should no longer lack it.

Donald Trump still has great power as the president of the United States. But as presidents go, he is very weak. His opponents — Democrats, independents and Republicans who understand the damage he is doing to the country — should be feeling energized.