That desire has played out over the last several years within a Republican Party that placated and indulged him, and accepted his money and support, seemingly not grasping how fervently determined he was to become a major force in American politics. In the process, the party bestowed upon Mr. Trump the kind of legitimacy that he craved, which has helped him pursue a credible bid for the presidency.

“Everybody has a little regret there, and everybody read it wrong,” said David Keene, a former chairman of the American Conservative Union, an activist group Mr. Trump cultivated. Of Mr. Trump’s rise, Mr. Keene said, “It’s almost comical, except it’s liable to end up with him as the nominee.”

Repeatedly underestimated as a court jester or silly showman, Mr. Trump muscled his way into the Republican elite by force of will. He badgered a skittish Mitt Romney into accepting his endorsement on national television, and became a celebrity fixture at conservative gatherings. He abandoned his tightfisted inclinations and cut five- and six-figure checks in a bid for clout as a political donor. He courted conservative media leaders as deftly as he had the New York tabloids.

At every stage, members of the Republican establishment wagered that they could go along with Mr. Trump just enough to keep him quiet or make him go away. But what party leaders viewed as generous ceremonial gestures or ego stroking of Mr. Trump — speaking spots at gatherings, meetings with prospective candidates and appearances alongside Republican heavyweights — he used to elevate his position and, eventually, to establish himself as a formidable figure for 2016.