Ten years ago, during the 2008 election campaign, a number of prominent commentators, including Jon Meacham and Karl Rove, repeated the claim that America is a “center-right country.” That view was plausible when George W. Bush was still president, but since then the center right has lost ground even within the party that has been its traditional home. The past decade’s developments raise the question of what place the center right has in the nation’s politics and what its decline says about America.

Barack Obama’s presidency set back the center right, but Donald Trump’s victory has had a far more devastating impact. As right-wing populist and nationalist leaders have risen around the world, one of their first priorities has been to clear the field of their right-of-center rivals. Mr. Trump has done exactly that by seizing control of the Republican Party and relentlessly attacking the few congressional Republicans who have defied him.

Mr. Trump’s purges continue the Republican march to the right that began in the 1980s. At that time, many leading Republicans were economically conservative but socially liberal or moderate. Often from elite backgrounds, they called for balanced budgets, free trade and other pro-business policies, while also supporting abortion rights, racial inclusiveness, immigration reform and environmental protection. Republicans had little hope of congressional majorities without them. But as the center of gravity in the party shifted south and west, the moderates became a dwindling and dispensable minority, increasingly forced to adapt to the new orthodoxy or to quit the party.

In the Senate, for example, the group of Republican moderates in the Wednesday Club, which had nearly two dozen members in the 1980s, had shrunk to five by 2008. Three of them — Lincoln Chafee, Jim Jeffords and Arlen Specter — became Democrats or caucused with them. Olympia Snowe retired, and Susan Collins, the last one in the Senate, may just have sacrificed her reputation as a moderate with her immoderate speech endorsing Brett Kavanaugh.