Donald Trump is on a collision course with his own party.

Republicans have publicly rebuked the president for threatening to shut down the government if they don’t fund construction of a southern border wall.

Building a wall between Mexico and the United States was Trump’s signature campaign promise. But it is an impractical plan that few lawmakers are willing to spend billions of dollars on.

When they return from summer recess in two weeks, Republicans will have about twelve working days to pass a budget that Trump must sign to keep the government running.

If Trump makes good on his threat (and he seldom does), it could trigger a civil war inside the party — which he is most likely to win.

Why Trump would prevail

David A. Hopkins, a political scientist, points out:

• Trump’s approval rating is sinking, but congressional Republicans are even less popular.
• The latter would enjoy the support of conservative media like National Review and The Wall Street Journal, but they are no match for aggressive pro-Trump outlets like Breitbart, Fox News and talk radio.
• Whereas most national Republicans have shied away from criticizing Trump for fear of alienating their base, the president never hesitates to berate his opponents. Nor does he feel restrained by the facts.
• If Congress blocks him, Trump could use executive action to claim accomplishments. Republicans in Congress cannot enact legislation without Trump’s signature — or support from Democrats.
• Trump is tapping into suspicions long held on the right that Republicans in Congress don’t mean what they say. They promised spending cuts when Barack Obama was president and had little choice but to also raise taxes. They promised to repeal Obamacare and failed.

At odds

Trump’s shutdown threat over the wall does not stand on its own. The former real-estate developer has been at odds with his party since the moment he announced his presidential candidacy.

In the last few days alone, he has reproached Senate leader Mitch McConnell for failing to protect him from an investigation into Russia’s role in the 2016 election and voiced support for Arizona senator Jeff Flake’s primary challenger.

The likes of McConnell lukewarmly supported Trump, expecting him to at least rubber-stamp Republican policies like health reform, tax cuts and deregulation.

But Trump doesn’t share his party’s priorities. He is not interested in the nuts and bolts of legislation. He only cares about being top dog.

Trump’s incessant need to dominate is a liability to Republicans who want to govern. But if they abandon him, they might not get anything done.