In 1976, when Reagan, the conservative insurgent, unsuccessfully challenged President Gerald R. Ford for the Republican nomination, the party platform was billed as a “moderate alternative” to the Democratic version, supporting “continuance of the public dialogue” on abortion and increased federal spending on the arts. Even four years ago, one major Republican candidate, Rudolph W. Giuliani, backed abortion rights.

The uniformity of views this year — forcing candidates to distinguish themselves by showing rigid fealty to conservative ideology — strikes some veteran Republicans as misguided and electorally dangerous.

“It is not true that there are big differences between these candidates,” said Vin Weber, a Washington lobbyist and onetime House Republican ally of Newt Gingrich who now backs Mr. Romney. “And it’s not true that the philosophical purity of the candidate is the key to beating Obama.

“Both those things make me nervous,” Mr. Weber said. He attributes the latter view to the “choice, not an echo” strategy that the conservative movement has championed ever since Barry Goldwater won the 1964 Republican presidential nomination — and carried only six states that November.

But the attempt to find wedge issues remains.

Mr. Romney and Mr. Santorum both support the repeal of President Obama’s national health care law. Mr. Santorum has criticized Mr. Romney for creating the Massachusetts plan that served as the federal model. Mr. Romney, meanwhile, recently blamed Mr. Santorum for playing a crucial role in the passing of the federal bill, because in 2004 he endorsed the re-election of Senator Arlen Specter, who later voted for the Obama health plan. (Mr. Specter, a rare Republican liberal, ultimately switched parties and was defeated for re-election as a Democrat.)

Mr. Romney and Mr. Santorum both supported the No Child Left Behind education law. But Mr. Romney, challenging his rival’s reputation for staunch conservatism, attacked Mr. Santorum for acknowledging that he had backed it only out of loyalty to President George W. Bush.

Mr. Santorum backed Mr. Bush’s expansion of the Medicare prescription drug program. Jim Talent, the former senator of Missouri who has endorsed Mr. Romney, recently slammed Mr. Santorum for that support on a conference call.