President Donald Trump is floating an idea that’s been kicking around Washington for years to get his $5 billion in border wall funding: the nuclear option.

Right now, legislation in the Senate requires a supermajority of 60 votes to overcome the filibuster, and this applies almost any legislation through the chamber. At the moment, there are only 51 GOP senators. And though their number will increase to 53 in the new year, that’s still well short of 60. So Republicans can’t pass an ordinary bill without getting Democratic support.

Trump repeated a suggestion he has made in the past on Friday — that the Senate should use the rarely imposed “nuclear option” to change its rules, and let bills advance from a majority vote alone.

Mitch, use the Nuclear Option and get it done! Our Country is counting on you! — Donald J. Trump (@realDonaldTrump) December 21, 2018

The way Trump sees it, the nuclear option is one way of getting his way on the wall. But Republicans are scared of what happens in the future, when they don’t control a simple majority of seats in the Senate.

The Senate’s supermajority requirement has been an enormous frustration for Trump during his term in office. Senate Majority Leader Mitch McConnell has already used the nuclear option once — back in April 2017, after Democrats filibustered Trump’s nomination of Neil Gorsuch to the Supreme Court, the Senate GOP went nuclear to get Gorsuch and any other Supreme Court nominees confirmed.

But defanging the filibuster for legislation is a far tougher sell in the Senate — and many senators have made clear they’re not interested in doing so.

McConnell Spox @davidpopp out with new statement regarding Trump call to nuke the legislative filibuster for the CR/border wall $ bill.



Translation: Not going to happen. pic.twitter.com/Duhndxb5Fk — Frank Thorp V (@frankthorp) December 21, 2018

Still, filibuster reform is not just some fringe Trump idea. Many Democrats, particularly from the more progressive side of the party, have also embraced the issue, particularly after frustrations passing major legislation during Barack Obama’s presidency. And there are arguments from good government reformers, too, that such a reform would make the Senate more democratically responsive and help presidents enact their agenda.

What is the nuclear option?

The Senate adheres to an elaborate set of rules and precedents about just what it can and can’t do. Those rules give the minority several tools to delay or block measures it doesn’t like.

However, a determined majority of the Senate can also vote to break — or change — the chamber’s rules. This is what’s known as the “nuclear option.”

Now, when the nuclear option comes up, it’s usually because senators want to limit or eliminate the power of one of those useful tools for the minority in particular — the filibuster. So let’s recap how the filibuster works.

If a bill or nomination comes for a final vote before the Senate, it can pass with a simple majority.

However, to actually get to that final vote, Senate rules require the chamber to approve what’s known as a “cloture motion” cutting off debate on the topic.

This is important because any member of the minority can try to block a cloture motion by using a filibuster.

And to overcome the filibuster for ordinary bills, support of a majority of senators isn’t enough. Instead, 60 Senate votes — three-fifths of the chamber — are needed.

Because of that higher threshold for success, in recent years the cloture vote has essentially become the real vote determining whether a bill or amendment will pass. It’s no longer just used in extraordinary circumstances — instead, it’s become the main way the minority flexes its muscles in the Senate.

The catch is that if the majority believes the minority is abusing its filibuster power — say, by blocking nominees who they think are qualified, or blocking every nominee for a certain post — they have the threat of the nuclear option in reserve.

So the Republican-controlled Senate of 2005 threatened to use the nuclear option because they thought Democrats were filibustering too many of George W. Bush’s appellate court nominees.

Then, the Democrat-controlled Senate of 2013 actually did use the nuclear option to effectively eliminate the filibuster for all nominations except Supreme Court ones. (This was in response to the apparent GOP desire to filibuster anyone Obama planned to nominate to the DC Circuit Court of Appeals, as well as what Democrats saw as excessive Republican filibusters against nominees generally.)

And finally, the Republican-controlled Senate of 2017 used the nuclear option to finally eliminate the filibuster for Supreme Court nominations too, so as to confirm Neil Gorsuch after Democrats filibustered his nomination. Because of that, it now only takes a majority vote of senators to confirm a Supreme Court justice.

But it still takes 60 to advance a bill — which is Trump’s problem.

How does the nuclear option actually work?

When then–Senate Majority Leader Harry Reid used the nuclear option in 2013, the specifics of what he did are complex. (They’re laid out in greater detail in this Congressional Research Service report.) But the basics were:

Reid raised a “point of order” that simply asserted that cloture votes for all nominations except the Supreme Court could pass with a simple majority, even though the Senate rules said otherwise. (This is the rules change he is trying to ram through.)

The senator presiding — the “chair” — ruled against him, because, well, that’s not what the rules said. The rules said you needed three-fifths, not just a majority, to pass a cloture motion.

Reid appealed to the full Senate to overturn the chair’s ruling.

A majority of senators (52 of the 55 Democrats) voted with Reid that the chair’s ruling should in fact be overturned.

And that was it. That vote effectively changed the Senate’s rules by overruling the chair to set a new precedent. From then on, cloture votes for all nominations except the Supreme Court could pass with a simple majority — meaning a filibuster from a minority of senators could no longer stop them.

In April 2017, McConnell followed essentially the same playbook — this time, with every Senate Republican joining him. No Democrats came along, but he didn’t need them.

So why won’t Republicans do it now for legislation, as Trump wants?

The mainstream of Senate opinion in both parties has tended to favor preserving the filibuster, for a few reasons.

For one, it’s a power senators can use when their party is in the minority. Senators enjoy being able to block bills they don’t like, in contrast to the largely powerless House minority party.

It’s also viewed as just the way the Senate does things, and has long done things. In fact, the filibuster threshold used to be higher — it took 67 votes to beat a filibuster until 1975. (However, reformers counter this argument by pointing that the use of the filibuster has greatly increased in recent decades.)

Many senators also believe reducing the filibuster threshold would damage what they envision as the Senate’s constitutional role as to “cool” majoritarian passions. Eliminating the filibuster would surely lead to more ideologically extreme measures on both sides passing, and moderates are temperamentally opposed to that.

A more cynical explanation is that the filibuster gives senators and party leaders “cover” from their respective parties’ activist bases, for why they haven’t yet managed to pass new laws that activists demand but that leaders view as too extreme or politically perilous. “Hey, we all voted for it, but we just can’t get around that darn filibuster,” senators can claim. “Just help more of our party get elected, and you’ll get what you want!”

Still, change is possible, as the recent uses of the nuclear option for legislation shows. Usually, what has to happen first is that one party becomes convinced the other is abusing the filibuster. That they’re taking advantage of it in an unfair way. And that’s the justification for going nuclear.