When Senator Mitch McConnell, the Kentucky Republican and longest-serving Senate leader, decided to step aside from his leadership role at the end of the year, it signaled the turning of a new page in the chamber.

But the intensifying battle to replace him, between Senators John Cornyn of Texas and John Thune of South Dakota and possibly others, is really a throwback to an earlier era, when leadership races in Congress were crowded and sometimes messy affairs featuring prominent figures and dueling factions.

For all the power they wield in Congress, Senate leaders have not had to fight too hard for their positions in recent years. Mr. McConnell, the current record-holder with almost 18 years at the top, did not face an opponent when he first won the job in 2006. He remained unchallenged until last year, when he had to fend off a weak coup attempt by Senator Rick Scott of Florida.

Before Senator Harry Reid’s retirement in 2017, the Nevada Democrat and party leader passed the reins seamlessly to Senator Chuck Schumer of New York. Mr. Reid himself had quickly sewn up the Democratic job when it suddenly came open in 2004.

But uncontested transitions were not always the norm. In years gone by, contenders for Senate leadership posts typically spent months maneuvering to secure secret commitments from colleagues who naturally all wanted something in return (and sometimes reneged on those commitments).

From now into November, every vote, statement and political move that Mr. Cornyn and Mr. Thune make will be analyzed in the context of the competition between them to become G.O.P. leader. That is more reminiscent of a pitched battle in 1984 that pitted some of the most powerful Senate Republicans against one another in a storied showdown to win the top job.

And while Mr. McConnell set off fears of a protracted battle by announcing in February that he would step aside at the end of 2024, the 1984 contest extended for more than double that amount of time. It dragged on for almost two years after Senator Howard H. Baker Jr., Republican of Tennessee and a legendary figure on Capitol Hill, announced in January 1983 that he intended to retire from the Senate and his leadership role.

“It was a much longer run,” Senator John Barrasso, Republican of Wyoming, said about the 1984 race compared with this year’s. “What we have here now, this year, is really not as extraordinary as you might think.”

Currently the No. 3 in Senate Republican leadership, Mr. Barrasso opted out of the fight for No. 1 and appears to have locked up the votes for the No. 2 position next year, moving up a notch. While weighing his prospects, the student of Senate history began researching past leadership elections to see what he could glean from them.

The 1984 race piqued his interest, because another Republican senator from Wyoming, Alan K. Simpson, was part of the maneuvering — and because so many top Republicans jumped into the fray.

“Five people got into the race to be leader and they were big names at the time, still big names around this place,” Mr. Barrasso noted. “There are members of the Senate who are here now who worked for those guys as interns.”

Jumping into the race were Senators Ted Stevens of Alaska, Bob Dole of Kansas, James McClure of Idaho, Richard G. Lugar of Indiana and Pete V. Domenici of New Mexico. Mr. Stevens, then the No. 2 Republican, was considered a formidable contender in the usual line of succession while Mr. Dole had been the party’s vice-presidential nominee in 1976. The men spent months quietly positioning themselves for the eventual face-off.

“Well, you go around and see everyone and you keep score,” Mr. Dole remembered in an oral history that aired on C-SPAN. “Some people, you know, they want to be your friend and they want to say, ‘I’m with you; you can count on me.’ How long — that’s the question.”

The leadership contest was as closely watched on Capitol Hill as the national elections that year. When the day came, Mr. McClure, the most conservative, was ousted first in the secret balloting as the party was aiming to move more toward the center — a marked contrast from today.

He was followed out of the running in consecutive votes by Mr. Domenici and then Mr. Lugar. It came down to Mr. Stevens, the fiery Alaskan who thought he had it wrapped up, and Mr. Dole, who ultimately won by three votes and ended up serving as leader until he resigned his Senate seat to run for president in 1996.

As is often the case, the insider race turned on issues beyond the candidates themselves. One factor working against Mr. Lugar, the chairman of the Foreign Relations Committee, was a fear that were he to be elected leader, the archconservative Senator Jesse Helms of North Carolina would become the top Republican on the panel, an outcome many wanted to avoid.

In New York Times coverage of the election, it was noted that one of those taking part as a newly elected member of the 1984 class was Senator-elect A.M. McConnell, not yet known as Mitch in the pages of The Times.

With many of the big party guns running for leader, Mr. Simpson, like Mr. Barrasso, opted for No. 2 and triumphed, only to be knocked off 10 years later in an upset by Senator Trent Lott of Mississippi — even though Mr. Simpson believed he had the votes to prevail and the backing of Mr. Dole.

“I thought I had it made when I ran again,” Mr. Simpson said in an interview. “I had about a three-vote cushion there and I lost by one vote to Trent.”

Democrats had some tough races of their own in 1988 and again in 1994, when Senator George J. Mitchell of Maine announced in March that he would not be returning, setting off a competition to be his successor.

Initially, the contenders were Senators Jim Sasser of Tennessee, the Budget Committee chairman, and Tom Daschle of South Dakota, a member of the party leadership. Then Republicans routed Democrats in an election-year tsunami that engulfed Mr. Sasser, who lost his own re-election before he could run for the Senate post.

Mr. Daschle briefly appeared home free, but Senator Christopher J. Dodd of Connecticut decided to challenge Mr. Daschle.

“It was old guard versus new members,” recalled Mr. Daschle, who was in the early years of his second term compared with his more senior colleague from Connecticut. “I went back to my supporters just to confirm they were going to be there to vote.”

In the end, Mr. Daschle defeated Mr. Dodd by a single vote, with the final one cast by Senator Ben Nighthorse Campbell, a Colorado Democrat who was shortly to become a Republican.

“It wasn’t exactly a mandate,” Mr. Daschle wryly noted.

When Mr. Daschle lost a re-election bid in 2004, Mr. Reid, who had been his No. 2 and had taken very good care of his colleagues on the Senate floor for years, had secured the votes to replace him before any potential opponents could do more than think about seeking the job.

Now it is Mr. Thune and Mr. Cornyn who must navigate the twists, turns and vagaries of internal elections and try to keep their supporters locked up for months. If past leadership fights have shown anything, it is that the unexpected is always just around the corner.