Many Republican officials believe that a contested convention, one that begins without delegates knowing who the nominee will be, is all but certain this July. Though Mr. Trump has a lead of 744 delegates to 559 for Senator Ted Cruz of Texas, neither man seems likely to secure an outright majority of 1,237 by the convention. And without a majority, the convention votes again — on second, third and subsequent ballots if necessary — until one candidate attains the 50 percent plus one that will make him the nominee. Most delegates are not required to support whomever their state’s primary- or caucusgoers did after the initial ballots.

Unlike the typical campaign effort, which involves trying to win over states or congressional districts or large blocs of voters, the convention involves a relatively tiny universe of people who will decide the candidates’ fate — 2,472 in all.

When the Ford and Reagan campaigns were engaged in a delegate-by-delegate fight for the nomination, they mainly relied on thick paper dossiers stuffed with every knowable fact about their targets, from whom they married to whether they liked to play bridge or do needlepoint in their spare time.

“It’s literally in the trenches, one person at a time,” said Mr. Black, who was just 28 at the 1976 convention. “We’re going to have to do it the old-fashioned way,” he added, “which is to make friends with people.”

In many ways, the first contested convention of the 21st century could hark back to the 19th and early 20th centuries, when nominees were often picked after many rounds of balloting. Assets like money and media exposure will matter less than a candidate and his team’s ability to connect on a human level with a few thousand delegates in a hall.

“You’ve got to know who they are, where they’re from, what they eat, what their hobbies are, where they went to college and on and on and on,” Mr. Spencer said in an interview by phone from Palm Desert. “That was true then. It’s true now.”