A week ago, things were going exactly the way Senate Majority Leader Mitch McConnell (R-KY), Donald Trump, and the Republican National Committee (RNC) had dreamed they might: Bernie Sanders was winning big in the Democratic presidential primaries and former Vice President Joe Biden’s campaign appeared to be stuck in neutral.

Then Super Tuesday came and shook up the Democratic race, making Biden the front-runner and Sanders on the outside looking in, down by 79 delegates with more tough contests for the Vermont Senator still to come in the weeks ahead.

Biden’s surge is good news for Democrats in what are known as “down-ballot” races for the U.S. House and Senate, who were worried that Sanders would win the nomination and be branded a “socialist,” which would have possibly kept control of the White House and Senate in GOP hands.

But with Biden in the lead and getting stronger, McConnell is facing his “worst nightmare,” according to A.B. Stoddard, associate editor for RealClearPolitics, who was asked by CNN host Jim Sciutto on Friday:

“Let’s talk down-ballot. Big argument and you heard this from sitting Democratic lawmakers worried about a Sanders nomination effect on down-ballot races. One said it will lose us twenty-five, thirty -that kind of thing. Now we’re seeing Biden surging. You see some of that fear being relieved and Steve Bullock jumping into the race, races where Democrats have a chance of getting — you know, bringing back the Senate. What do you see the down-ballot races stand today?”

That led Stoddard to note:

“This is Mitch McConnell’s worst nightmare.

“Donald Trump may not appreciate the threat to the Senate Republican majority, but [Senate Majority Leader] Mitch McConnell certainly does. [Sen.] Thom Tillis in North Carolina, he’s not only below his Democratic rival, it is now officially a tossup race. [Sen.] Martha McSally in Arizona, below her Democratic rival. They’re going to have to spend tons of money on John Cornyn in Texas and [Iowa Senator] Joni Ernst.

“This is becoming expensive, this is a real threat and with Biden at the top of the ticket, the majority is in question.”

McConnell himself is also in a deep hole when it comes to his own reelection effort. Recent polling shows that only 24 percent of Kentuckians have a favorable view of him. 44 percent say they see McConnell unfavorably. That’s a 20-point spread, and it’s hard to win an election with numbers that low. Especially when you consider that he’s facing a Democratic challenger, Amy McGrath, who’s a war hero and is raising enormous sums of money for her bid to unseat McConnell.

What a difference a week makes! For Mitch McConnell, this could wind up being the week from hell he never thought he’d be facing. But he is, and things are getting worse by the day for him and Republicans.