Mitch McConnell's decision to step aside as US Senate Republican leader raises questions about whether the chamber long known for being slow and deliberative will be swept up in the heated populism of Donald Trump.
In recent weeks, the Senate has taken on the more raucous atmosphere of the House of Representatives, with Republican hardliners fighting a bipartisan bill backed by Mr McConnell to toughen border security and provide more aid to Ukraine.
With Mr McConnell, 82, planning to hand over the mantle to a successor after the November election, hardliners hope their time has come to lead the chamber.
Senators serve for six years and represent entire states, unlike their House counterparts who are up for election every other year and represent districts that are often drawn to insulate them from pressure from the other party.
Lawmakers and aides say that has slowed the rising tide of populism in the Senate, though some said times are changing.
"We're seeing this populist brand of leadership rise to the top," senator Roger Marshall told reporters. "More of us that have been elected more recently seem to be a little bit more populist driven."
Unstable Republican leadership
The narrow 219-213 House Republican majority has proven difficult to lead, having ousted former speaker Kevin McCarthy last year and repeatedly bucked his successor, Mike Johnson.
Mr McConnell has held a far tighter rein, drawing opposition from hardliners, some of whom urged him to step aside immediately.
Lawmakers including senator Josh Hawley said a new Republican leader would have a better relationship with Mr Trump, who was president from 2017-2021 and is the frontrunner for the party's 2024 presidential nomination.
"My question: is why wait so long? I mean, November is a long time away," Mr Hawley told reporters. "We need new leadership now. But this is better than nothing."
While Republican hardliners railed against Mr McConnell as insufficiently allied with Mr Trump, Democrats long reviled him for hardball tactics with them. Mr McConnell embraced the nickname the "Grim Reaper" for his success in blocking their priorities even when he was in the minority, and for helping Mr Trump secure a 6-3 conservative Supreme Court majority that expanded gun rights and ended a nationwide right to abortion.
Foreign policy implications
A populist tilt in Republican Senate leadership could have serious implications for US foreign policy, including support for Nato and aid to Ukraine's war effort against Russia, especially if Republicans capture the White House and a Senate majority in the November 5th election.
"On foreign affairs, there's quite a big divide between the Trump wing of the party, which is the dominant wing, and the McConnell wing," said Senate republican Mitt Romney, a Trump critic.
Bipartisanship could also be harder to pull off.
In recent weeks, hardline Senate Republicans worked to jettison a bipartisan deal to address the flow of migrants across the US-Mexico border, after Mr Trump opposed it, and worked against efforts to deliver emergency aid to Ukraine, Israel, Taiwan and other US allies.
Republican senator J.D. Vance, a staunch Trump ally, told reporters that the Trump wing better reflects the Republican Party and the views of Republican voters on critical issues than does the McConnell wing.
"There was going to be a really contentious leadership election if he (McConnell) had run (for leader) again in November. But it looks like we're going to avoid that," Mr Vance said.
With no serious challenge to Mr McConnell's leadership since he was elected to the job 17 years ago, it was unclear how many senators would vie to replace him.
Three lawmakers - known as the three Johns - have been widely seen as contenders.
Senators John Thune and John Cornyn have both worked closely with Mr McConnell as his top lieutenants, while Senate conference chair John Barrasso, who has a harder conservative edge, is also no stranger to the leadership team.
Other candidates, including from the far right, seem all but certain.
Senator Rick Scott, who was defeated soundly by Mr McConnell in late 2022, had plenty to say about the need for new leadership.
"There's a better way to run the Senate," Mr Scott said. "We're not dealing with the deficit. We've got a border that's completely wide open. There's a lot of issues we've got to deal with."
Insiders say the job of getting elected party leader depends less on political stances than on assuring rank-and-file senators that their interests will be represented.
Former McConnell aide Josh Holmes said the next Republican leader should seek to acquire Mr McConnell's knack for insulating the conference from outside pressures.
"On a lot of occasions, he said he's more proud of his enemies than his friends," Holmes told Reuters. "I'm sure there will be growing pains, as there were with us when we started. To get really good only comes with doing it."
