Two weeks after Donald Trump issued an emergency declaration, announcing plans to redirect funds to border barriers in defiance of Congress’ wishes, the U.S. House passed a bipartisan bill to block the president’s gambit. The measure is now pending in the Senate, where the prospect of another Trump defeat is very real.

Remember, this isn’t one of those bills Senate Majority Leader Mitch McConnell (R-Ky.) can just ignore. The Republican-led chamber will have to vote on the measure, and it cannot be filibustered. If the bill gets 51 votes, it will pass.

And as things stand, I’m reasonably sure the resolution will have the support it needs.

The magic number in this case is four: if four Senate Republicans vote for the measure, it’ll pass. Early this week, three GOP senators – Thom Tillis (N.C.), Susan Collins (Maine, and Lisa Murkowski (Alaska) – expressed support for the proposal. Yesterday, a possible fourth stepped up.

Sen. Lamar Alexander (R-TN), a retiring lawmaker who has already expressed his distaste for President Donald Trump’s national emergency, hammered home his point Thursday, calling the executive runaround “unwise and unnecessary.” “I support what the President wanted to do on border security, but I do not support the way he has been advised to do it,” Alexander said, careful to use the passive voice. “It is unnecessary and unwise to turn a border crisis into a constitutional crisis about separation of powers when the President already has congressional funding authority to build a 234 miles of border wall that he requested in his January 6 letter to the Senate.”

To be sure, Alexander did not explicitly say he’ll vote “yes.” But the retiring Tennessean recently criticized Trump’s gambit, and if Alexander were planning to reverse course, he would’ve done the opposite of what he did yesterday. Instead, the Republican sent another shot across the White House’s bow. Other GOP senators are sending similar signals.

In fact, the whole debate is shifting quickly. With Trump facing an increasingly likely defeat, the Wall Street Journal reported overnight that Senate Republicans are warning the president “against moving forward with his national-emergency declaration to build a southern border wall and pressuring him to restrict the project’s funding to less controversial sources.”

Or put another way, GOP leaders on the Hill are effectively telling Trump, “To avoid embarrassment, you’ll need to change course.”

For now, the president is ignoring the advice. Trump is publicly predicting that his partisan allies will stick with him, and he told Fox News this week that those who vote to block his “emergency” scheme are putting themselves “at great jeopardy.”

Looking ahead, the Senate has to act within 18 days of the House passing its bill, and under the National Emergencies Act, which is the law Trump is utilizing for this gambit, those are 18 calendar days, not days in which Congress is in session.

That gives the upper chamber a deadline of March 16. If Trump retreats, as some Republicans are recommending, the vote will be a moot point.

* Postscript: Before anyone sends me angry emails, I know that if Congress passes the Democrats’ resolution, Trump will veto it, and there probably won’t be enough votes to override him. That said, a defeat on this would be a brutal embarrassment to the president, and if both the House and Senate were to pass a resolution rejecting Trump’s policy, that’s likely to matter to judges assessing the legality of the White House’s dubious scheme.