Of course he was:

Romney’s endorsement of Rubio, which follows those by fellow 2008 presidential contenders Rudy Giuliani and Mike Huckabee, was a long time coming. “Why now? The veto that Governor Crist made of the education-reform bill really tipped the balance for me,” Romney explains. “I have to admit that I was very concerned when I saw Governor Crist support the Obama stimulus. I was very much inclined to support Speaker Rubio when I watched the debate on Fox, the one with Chris Wallace. But the decision to actually step in occurred when I had heard that he had vetoed the education-reform bill.”