That word again. For the right, the establishment has even more complex associations than for the left, because the founders of modern conservatism were the first to speak routinely of a “liberal establishment” a like-minded elite that was said to exert undue influence over Congress, academia, the news media and more. (In his essay, Mr. Rovere noted tartly that conservatives of his time, including the editors of National Review, evidently believed the establishment to include “just about everyone in the country except themselves.”) Many of the conservative groups and leaders who oppose Mr. McCain are the same ones who decades ago felt that their own party’s establishment was dominated by a few secret “kingmakers” who steered presidential elections toward moderates like Dwight D. Eisenhower, as Phyllis Schlafly, the founder of the Eagle Forum, wrote in her influential book “A Choice Not an Echo.”

Insurgents like Mrs. Schlafly emerged as a potent force in the 1964 primary, gaining the nomination for Barry M. Goldwater, and went on to form the conservative establishment that dominated the Republican Party for the next 40 years. They still see themselves as indispensable kingmakers without whom no Republican can win the nomination, let alone the White House. As a result, Mr. McCain has emerged as a genuine threat. Should he win the nomination over their opposition after all, the kingmakers would be dethroned.

“What goes around comes around,” said Morris P. Fiorina, a senior fellow at the conservative Hoover Institution and a professor of political science at Stanford University. “It’s a self-appointed establishment to a great extent, and I think all along they overestimated their own importance.”

The closer you look for signs of either party’s establishment at work, it seems, the more the very idea seems to crumble and dissolve. On both sides of the political divide, the people and institutions once considered integral to the establishment have become too weak or fractious to deserve the term.

On the left, for example, labor unions were once overwhelmingly powerful in the Democratic Party. Today, they remain a major force in party affairs, but not a very monolithic one. In 2005, several significant unions split from the A.F.L.-C.I.O. after disagreements over organizing strategy and formed a new group, the Change to Win Coalition.

Perhaps the only vestige of establishment strength in the Democratic primaries are the so-called superdelegates  sitting governors, senators, state chairmen and the like, who make up about a fifth of the delegates and can back whomever they like. They were created after the divisive 1980 primaries, with the idea that in close contests, party leaders could quickly coalesce behind one candidate, heading off a nasty nomination fight.