Today the House will vote on Joaquin Castro’s resolution to cancel Donald Trump’s declaration of emergency at the border. With a House majority secured, Nancy Pelosi needs to get as many Republicans to vote in favor of nullifying Trump’s action — both in the House and in the Senate. Thus the mission of Republican leaders in both chambers has shifted from defeating the bill to ensuring that a veto will succeed:

Congress braced Monday for an unprecedented effort to overturn a presidential emergency declaration, as Republicans worked to limit defections on the eve of a critical House vote while Democrats framed the issue as a constitutional showdown.

Partisans on both sides unleashed sharp new rhetoric ahead of Tuesday’s vote on a Democratic-authored resolution that would nullify President Trump’s declaration of a national emergency at the U.S.-Mexico border. Congress has never before sought to cancel a national emergency declared by the president since passage of the National Emergencies Act in 1976.

The resolution is expected to pass the House easily with unified Democratic support. But GOP leaders were urging their members to oppose it, aiming to keep the final tally low enough to demonstrate that Congress would be unable to overturn the veto that Trump has threatened.

Let us pause for a moment to reflect on just how “unprecedented” this action truly is. Presidents have issued 59 previous declarations of emergency without any congressional action to nullify it. In fact, 31 of those remain in effect to this day, with the president holding all the emergency powers on those issues, with some of these emergencies stretching back decades. That includes the very first emergency declaration issued, in 1979 by Jimmy Carter over the Iranian hostage crisis. The hostages came home in January 1981, but Congress has never bothered to act to limit the presidential authority and to take responsibility for that and all of the other “emergency” issues.

So now Congress is acting in an “unprecedented effort” after passing the buck for 43 years. Whatevs. When will Congress get to other 31 and genuinely protect their constitutional prerogatives? In another forty years?

Anyway, at least a few Republicans will cross the House aisle tonight for legislative honor. When the bill comes to the Senate, at least three Republicans have explicitly declared their support for the bill. One more and it will pass, forcing Trump to issue the first veto of his presidency. That’s the moment for which Mitch McConnell is hoarding his political capital rather than the first vote, which looks hopeless:

Senate Majority Leader Mitch McConnell (R-Ky.) isn’t going to use political capital to fight a Democratic-sponsored resolution disapproving of President Trump.

Instead, the GOP leader will bide his time and pick his battles carefully, avoiding a confrontation with fellow Republican senators who think Trump’s use of the emergency declaration to build border barriers is a policy mistake that sets a bad precedent. …

Already, three Senate Republicans — Susan Collins (Maine), Lisa Murkowski (Alaska) and Thom Tillis (N.C.)— say they intend to support the disapproval resolution, and at least a half dozen more are a threat to do the same. Two other likely defectors are Sens. Cory Gardner (R-Colo.), who faces a tough reelection in a state won by Democrat Hillary Clinton, and Lamar Alexander (R-Tenn.).

Tillis warned the emergency declaration would “set a new precedent that a left-wing president would undoubtedly utilize” while Alexander, who is retiring, called it “unwise” and “inconsistent with the U.S. Constitution.”

This message is brought to you by the numbers 4 and 20. The former is the numbers of Senate Republicans that Schumer needs to pass the resolution; the latter is how many Schumer needs to sustain the veto. It’s all but certain that Schumer will get to four, and with it do damage to Trump’s case for an emergency in the public sphere, if not the federal district courts in which lawsuits will be heard. (The appellate courts are more likely to shrug off the what-is-an-emergency question as inherently political and focus on the statutory grant of authority in the National Emergencies Act, however.)

There is no way that Schumer will get to 20 on a veto override, though. McConnell won’t need to twist arms to limit aisle-crossers well below that threshold. There simply aren’t that many vulnerable Republicans in the Senate, and for most of them their next election will take place too far out for this to be a problem anyway. There’s no point in alienating friends and colleagues over the inevitable as long as Trump’s willing to use the veto — and he’s not only willing but champing at the bit to have this fight.