Now that the Republican Party is in an important transition, how well does its new identity fit? Is it comfortable in its blue collars, or does it miss the college repp ties and loose Peter Millar golf shirts with their micro-mesh pique weave, the staples of its familiar, earlier wardrobe?

American political parties are always evolving, with the Republicans moving from the “bloody shirts” of the 19th century to the Dickies short-sleeved work shirts of the 21st. They change their war cries and their wardrobes with the fashions, and the passions, of the time.

The Democrats after the Civil War, for example, were a conservative Southern party rooted in the nation’s small farms while the Republicans were the party of industrialization, rooted in factories and the new booming urban areas. Neither party of that period bears any resemblance to its current profile.

But those changes, visible from a long perspective but often difficult to discern in real time, customarily take decades — but sometimes, prompted by a domestic crisis such as a severe economic downturn or a disruptive force such as Donald J. Trump, come in a frantic lurch.

The Republicans of 1936 were not substantially different from the Republicans of 1976, a 40-year period of stability in their identity. But add on another 40 years, and it is incontrovertible that the Republicans of 1976 and the Republicans of today have very little in common.

It is arguable that Gov. Alf Landon of Kansas, the 1936 GOP nominee, differed only slightly in tone and substance from President Gerald R. Ford, the party’s candidate four decades later. But the difference between Mr. Landon and Mr. Ford is far smaller than the difference, also measured in 40 years, between the 38th president and Mr. Trump, the 2016 nominee.

Both Mr. Landon and Mr. Ford were conventional political figures, and indeed Mr. Landon’s daughter, Nancy Landon Kassebaum, was a thoroughly conventional moderate GOP senator from 1978 to 1997 and she married Sen. Howard H. Baker Jr., who, as the Senate majority leader for four years and White House chief of staff for 17 months, was the very definition of a conventional political figure.

Now take the difference between Mr. Ford — who served in the House, vice presidency and the White House — and Mr. Trump.

Though both considered themselves insurgents, Mr. Ford’s rebellion, which consisted of toppling an established Republican incumbent, Bartel J. Jonkman, in 1948 and then unseating the House GOP leader, Charles A. Halleck, 15 years later, was within the customary bounds of political activity and never caused a crisis of identity within the party.

Mr. Trump, a Manhattan businessman with no political experience, has almost nothing in common with Mr. Ford, who was a veteran politician hand-picked by the ultimate veteran politician, President Richard M. Nixon, to serve as vice president as the Nixon White House was convulsed in the Watergate crisis and the likelihood grew that the president might be forced from office. Mr. Ford was a product of the Midwestern Republican outlook prevalent, then as now, in Grand Rapids, Mich. His rebellion against Mr. Halleck was an insiders’ revolt, not an attack on the values of the party or an outsider’s effort to reshape the party’s identity.

Mr. Trump’s assault on the Republican establishment is different in tone and meaning from the comparatively innocuous insurrections of Mr. Ford.

The irony is that the new Republican Party — or at least Mr. Trump’s Republican Party — has many of the cultural and demographic attributes of the party that the late Rep. Jack F. Kemp of New York, and by extension his protege, House Speaker Paul Ryan of Wisconsin, envisioned as an earlier battle for the soul of the GOP unfolded.

Mr. Kemp looked to broaden the Republicans’ appeal, much the way Dwight Eisenhower sought to broaden the appeal of a Republican Party that took its identity from Sen. Robert Taft of Ohio, known as Mr. Republican. He wanted the Republicans to welcome blue-collar voters who were not being served by — and who were being taken for granted by — the Democrats, who had claimed their allegiance since 1928, when Gov. Al Smith of New York was the party’s nominee, and who have held it firmly since 1932, when Gov. Franklin Roosevelt won the party’s nomination and then the White House.

Mr. Trump’s Republicans are open to blue-collar voters and, despite the nominee’s own wealth and his comfort in golf shirts — his, manufactured by Fairway and Greene and available in four “Trump Signature Solid” colors, are on sale for $79.99 in the pro shop of Trump National Golf Club in Los Angeles — are skeptical of the country-club Republicanism that was for many years an element of the party’s identity.

“It is true that Jack Kemp and Donald Trump share the idea that blue-collar voters belong in the Republican Party,” said David Smick, Mr. Kemp’s Capitol Hill chief of staff from 1979 to 1984. “But Kemp was a free-trader and liked the innovative, risk-taker approach to economic growth.”

The change in the character of the party’s profile produced by Mr. Trump is far more dramatic than the movement from Mr. Taft to Gen. Eisenhower, or even the movement, in Great Britain’s Conservative Party a few years later, from Harold Macmillan and Alec Douglas-Home (both Etonians) to Edward Heath and then to Margaret Thatcher (the first ridiculed as “Grocer Heath” for his 1962 Common Market negotiations on food prices and the second a Grantham grocer’s daughter).

The Trump revolution undermined the GOP’s comfortable self-image in two ways. The first is the composition of the Republican coalition, which traditionally included few blue-collar voters. The second is the evolution of the GOP into a disciplined party of conservatism.

The latter is why Ben Shapiro, writing in the conservative National Review this month, deplored what he described as Mr. Trump’s “attempt to turn the conservative movement into a nationalist populist one,” adding, “If you believe that the only solution to America’s problems is true conservatism, your greatest fear is not a Hillary presidency: It’s the perversion of the conservative movement itself, the corruption of conservatism in favor of power.”

And so the struggle continues, not only for the White House but for the character of the Republican Party. The former will be settled in November. The latter will be unsettled for years.