His border order will divide Republicans and may be stopped in court.

The White House said Thursday that President Trump will sign a border-security funding bill but also declare a national emergency to spend even more to build his wall at the Rio Grande. The emergency declaration will please his most ardent supporters, but Mr. Trump is setting an unfortunate precedent—and judges could tie up his wall in court for years.

Mr. Trump had little choice other than to sign the spending bill or see the government shut down for the second time in a month. He boxed himself in by saying in December that he’d gladly take ownership of a shutdown, only to discover that his poll numbers fell further the longer the January closure went on. Republicans in Congress bailed him out by getting at least $1.38 billion for border funding, enough for about 55 miles of fencing.

Yet rather than declare partial victory and fight again in the next budget, Mr. Trump will now test the limits of his executive power.

As a political matter, Congress may respond by passing a resolution in both houses to override his emergency declaration. A simply majority would suffice in both houses under the emergency law, and there may be enough Senate Republicans to get to 51 votes. Mr. Trump can then veto, and he’s unlikely to be overridden. But this would still be a political defeat that unites Democrats and divides his own party. We’ve argued that Mr. Trump might win his emergency gambit if the case goes to the Supreme Court, but it is a close call and he is taking a big legal risk. Property owners affected by the wall will sue, and the House of Representatives will surely sue as well ...

The plaintiffs will likely find some federal judge to enjoin the emergency declaration, and the Ninth Circuit Court of Appeals may well agree. If the Supreme Court doesn’t step in to let the President proceed while lower courts consider the merits, Mr. Trump will be stymied well into 2020. If he ultimately loses in court, Mr. Trump will have hurt his own standing and the power of his successors. This was the mistake Mr. Obama made so often when he took legal shortcuts to impose a policy he couldn’t win politically on Capitol Hill. Mr. Trump could find himself running for re-election having lost his wall in court and Congress. Constitutional conservatives should also worry if Mr. Trump wins in court. A precedent will be set that future Presidents could use to impose their own priorities despite a reluctant Congress. If climate change will end life as we know it in 12 years, why not impose part of the Green New Deal?

Mr. Trump’s obsession with building a wall has caused him no end of political grief. He’ll be lucky if this emergency declaration doesn’t end the same way.

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