"Short of impeaching him how would they do that?"
Please read following from the Constitution.
Article II, Section 1, Clause 6: In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the VicePresident, and the Congress may by Law provide for the Case of Removal [emphasis added], Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
"Cruz wants to be president."
Trump wants to be president.