“The Republicans’ presidential nominee must build a winning coalition that extends beyond the Republican base,” said Representative Charlie Dent, Republican of Pennsylvania. “Running a campaign and expanding the coalition is an exercise in addition, not subtraction.”

Senator Ted Cruz, the Texas Republican who has been chasing Mr. Trump in the polls in early primary states, is popular among religious conservatives and hard-right Republican voters. But Mr. Trump’s unusual bloc, such that it is, pulls in registered Democrats and other nontraditional Republican primary voters. Indeed, some polling has shown overlap in views among those who favor Mr. Trump and Senator Bernie Sanders of Vermont.

“Ted Cruz is a rigid ideologue,” said Mr. Dent, who is not supporting Trump and has yet to endorse anyone. “Donald Trump is ideologically scattered and malleable. In my view, a more rigid ideology would have a much harder time assembling a winning general election coalition than the less doctrinaire candidate.”