From the New York Times:
Mr. Crist, a two-term governor who once seemed to be on an easy path to winning the nomination and, perhaps, the Senate seat in the fall, has struggled to compete with the momentum and fund-raising generated by Mr. Rubio. Mr. Crist’s candidacy has foundered, and with an April 30 filing deadline looming, Republicans have grown increasingly worried that he would drop his party affiliation and run as an independent.