Mitt Romney will not be running for President. He had previously run for president in 2008 and 2012 (when he was to Republican Nominee).
From the Washington Post:
Romney insisted that he would have had enough support from potential donors to be “more than competitive” and that the positive reaction he heard from Republican activists was “surprising and heartening.” He noted that he led the GOP field in recent public polls.
“I am convinced that we could win the nomination, but fully realize it would have been difficult test and a hard fight,” Romney said in remarks he delivered by phone from New York with his wife, Ann, by his side.
No word on whether this decision was because of a call to be mission president.