WASHINGTON — What's next for President Donald Trump?

Expect a massive victory lap when he rallies in front of supporters in Kansas on Saturday night, which is likely to be just hours after the Senate's final vote to confirm his Supreme Court nominee, Brett Kavanaugh.

And it won't be a hollow victory lap: Friday marked a stunning comeback for Kavanaugh and an extraordinary political win for Trump.

One source close to the confirmation process told NBC News that the White House counsel's office was convinced it did enough to provide wavering senators the political cover they needed to vote "yes" -- by allowing the FBI to reopen the background check and by instructing Kavanaugh to write The Wall Street Journal op-ed reasserting his independence.

As for the mood inside the West Wing — "euphoria and relief," as one White House ally described it.

All afternoon Friday, White House officials seemed cautiously optimistic of the outcome: Unwilling to begin celebrating before they crossed the goal line, but feeling positive overall. White House officials took as a very good sign the lunch between Sen. Susan Collins, R-Me., and Senate Majority Leader Mitch McConnell, R-Ky., just hours before she was set to announce her decision on the Senate floor.

And when Collins, who was the last undecided GOP senator, finally announced she would vote Saturday in support of Kavanaugh — all but ensuring his confirmation — cheers broke out in the White House press offices.

All week, the White House has been in the position of spectator — on account of Trump's fraught relationship with Collins and Sens. Jeff Flake of Arizona and Lisa Murkowski of Alaska — but they've been in the driver's seat when it came to public opinion.

The president, while amping up the pressure via tweets and public comments, deployed the core tenets of Trumpian politics. He set up a binary choice for his base, essentially telling Republicans: You're with us, or you're with the obstructionist Democrats.

The Kavanaugh bet appears to be paying off. As one White House ally put it: The president "picked a winner. And he won."

But will that win be reflected in the midterms?

Some Republicans hope that the fierce Kavanaugh battle keeps GOP enthusiasm high until the November elections. But how does Democratic enthusiasm play into it? Remember: Court battles have largely been an asymmetrical political issue — extraordinarily mobilizing for Republicans but less so for Democrats.

The Kavanaugh fight might change that.