Until recently, the group had focused on Senator Marco Rubio of Florida. “For months, until about two or three weeks ago, every single person that I encountered in the Republican Party would begin every sentence the same way: ‘When Trump is gone,’ ” Ms. Conway said. “That itself was a flawed premise.”

.. Yet Mr. Cruz has been dealing with more than Mr. Trump. Mike Huckabee and Rick Santorum continue to attack him, and the state’s six-term governor, Terry E. Branstad, urged Mr. Cruz’s defeat, part of an onslaught from allies of the ethanol industry.