President-elect Donald Trump walks with his wife, Melania, Vice President-elect Mike Pence and Senate Majority Leader Mitch McConnell to a meeting in the U.S. Capitol on Nov. 10. (Shawn Thew/European Pressphoto Agency)

Of all the promises made on the campaign trail, President-elect Donald Trump’s vow to pass a constitutional amendment to impose term limits on members of Congress might be the most daunting.

Senate Majority Leader Mitch McConnell (R-Ky.) dismissed the idea out of hand the day after Trump’s stunning victory. A few days later, House Speaker Paul D. Ryan (R-Wis.) gave the proposal a tepid endorsement as he indicated it would be up to a House committee to consider Trump’s proposal.

The reticence of both Republican leaders on the issue is not surprising, given their long tenures in Congress.

McConnell has served in the Senate since 1985, and he is one of five sitting senators to have served more than three decades. With almost 18 years under his belt on Capitol Hill, Ryan would essentially be booted out of office under almost every term-limit proposal that has been floated in the past 25 years.

President-elect Donald Trump and House Speaker Paul D. Ryan (R-Wis.) pose for photos after a meeting in the speaker’s office on Capitol Hill. (Alex Brandon/AP)

Democrats generally oppose term limits, making it difficult to see a path toward the two-thirds supermajority required to pass a constitutional amendment that would get sent to the states for ratification.

What remains to be seen is whether Trump uses the presidential bully pulpit to continue pressuring Congress to adopt the idea, along with other ethics and lobbying reform proposals he unveiled last month in a campaign speech in Colorado.

“If I’m elected president, I will push for a constitutional amendment to impose term limits on all members of Congress,” Trump said at the time. “They’ve been talking about that for years. Decades of failure in Washington and decades of special interest dealing must and will come to an end.”

[Trump calls for term limits to ‘end cycle of corruption’ ]

The resurrection of term limits is part of Trump’s “drain the swamp” agenda, aimed at cleaning up what he sees as a rigged system in Washington ruled by lawmakers with close bonds to lobbyists. But if Trump follows through with some of his proposals, and continues his fiery anti-establishment campaign rhetoric, he could end up alienating the very Republicans he needs to help pass his other ambitious proposals on taxes and border security.

Some Trump allies are more interested in satisfying the president-elect’s campaign pledge to limit House members to just three two-year terms and senators to two six-year terms.

“Once President-elect Trump takes the oath of office, he must take action on this key promise to the American people,” Philip Blumel, president of U.S. Term Limits, a leader of the movement for several decades, said after the elections. “This is a sure way to help unify a divided nation.”

The term-limits movement gained momentum in the late 1980s and early 1990s, as 23 states passed legislation trying to impose a limit on how long members of Congress could serve. But the Supreme Court struck down those laws in a ruling making clear that the only limits to congressional service were spelled out in the Constitution, which would have to be amended to pursue change.

[Term limits are probably never going to happen]

The term-limits movement, which drew more support from conservatives than liberals, started its growth during an era when Democrats had a powerful hold on the congressional majorities — controlling the House from 1955 to 1995, while holding the Senate for all but six of those 40 years.

But when Republicans retook the congressional majority in the 1994 midterms, a new era of competition took hold. The Senate majority has flipped four times in the past 16 years, and the House majority has changed hands twice in the past decade.

That took much of the wind out of the sails for term limits.

“We have term limits now — they’re called elections,” McConnell said the day after the election. He said there would be no consideration of the proposal in the Senate.

Of the 100 senators, 64 have served in their seats less than 10 years, marking a generational transformation that has happened just twice in the past 100 years, first after World War II and then in the years after the Watergate scandal.

A decade ago, 17 senators had served more than 25 years in the chamber; today, nine have served at least a quarter-century.

And even though incumbents still win more than 90 percent of their reelection races, almost no one is safe in this turbulent political era. McConnell has faced two difficult elections in his most recent campaigns, 2008 and 2014, and outgoing Minority Leader Harry M. Reid (D-Nev.) stood down a difficult challenge in 2010 and would have faced another this year had he not opted for retirement.

Even Republicans in the most conservative states no longer feel entirely safe, because their party’s deepest conservative activists have regularly supported primary challengers to those seemingly entrenched lawmakers. Earlier this year, Sen. Richard C. Shelby (R-Ala.), a 30-year veteran, spent more than $10 million to defeat a primary challenge from the right.

The House has had a similar turnover. More than 240 of the 435 current members of Congress have served since January 2009, when President Obama first took the oath of office.

Embarking on his 10th House term in January, Ryan has a more complicated view of term limits. “I’ve always supported term limits,” he said during last week’s weekly news conference. “I’ve long been a fan of term limits. I don’t know where other members stand, but I’ve always been in favor of that.”

Ryan repeated this view over the years, including during a town hall in 2012 when he was the Republican vice-presidential nominee. “I’ve always believed that this should be something that you serve [temporarily], not for an entire lifetime,” he said.

But Ryan also said that states benefit when they have lawmakers with real clout, and that the only way to enact term limits was through a constitutional amendment defining lengths of service for all lawmakers.

The speaker said last week that the issue would be up to the House Judiciary Committee to handle, declining to specify whether such legislation would ever get a vote in the full House.

Trump could decide to try to shame lawmakers into at least voting on a proposal. He was the least popular person to ever win a major party’s presidential nomination, but the public disgust with Congress is epic, giving it an approval rating that has hovered in the low teens for six years now.

“We’re going to put on term limits, which a lot of people aren’t happy about, but we’re putting on term limits,” Trump said in an interview with “60 Minutes” that aired a few days after the election.

But it is unclear how potent this issue is.

Of all the Senate campaigns in 2016, just one candidate regularly touted support for term limits: Rep. Ann Kirkpatrick (D-Ariz.), who used the issue against Sen. John McCain (R), a 30-year veteran of the chamber.

McCain, 80, won his sixth term in the Senate by more than 12 percentage points.