Democrats’ attempts to circumvent the filibuster to pass a major voting rights bill are “a rule-breaking power grab” that would not be “harmless” or “cost-free,” Senate Minority Leader Mitch McConnell (R-Ky.) said Tuesday according to ABC News.

But what exactly are Democrats proposing?

Senate rules stipulate that 60 votes are needed to end debate on a bill — a procedure known as “cloture.” As long as debate remains open, senators can filibuster, delaying or even preventing the passage of the legislation in question.

But, the Senate also has the power to change its own rules at any time with a simple majority vote.

In this case, changing the rules could mean a one-time exception limited to this particular bill — as Sen. Tim Kaine (D-Va.) has suggested — or a rule change allowing cloture by simple, 51-vote majority for all future voting rights bills. Lowering the cloture threshold in this manner is known as the “nuclear option.”

It’s unlikely to happen. Democratic Sens. Joe Manchin (D-W.Va.) and Kyrsten Sinema (D-Ariz.) both oppose the rule change, leaving Democrats with only 48 votes.

If it were to happen, though, it would be the third use of the “nuclear option” in Senate history.

The first was in 2013, when then-Majority Leader Harry Reid (D-Nev.) allowed cloture by simple majority “for all nominations other than for the Supreme Court.” This change enabled Reid to overcome Republican filibusters that were holding up several of then-President Barack Obama’s nominees, NY1 reported.

At the time, McConnell threatened, “You’ll regret this, and you may regret this a lot sooner than you think.”

McConnell made good on his word in 2017 when he expanded Reid’s carveout to include Supreme Court nominees. This ended what McConnell called the “first, and last, partisan filibuster of a Supreme Court justice,” according to NPR. Democrats filibustered the nomination of Neil Gorsuch after McConnell refused to hold confirmation hearings for Obama nominee Merrick Garland.