While the field for the Republican presidential nomination is crowded and up for grabs, many pundits and politicos are ready to proclaim a front-runner for the ticket’s second spot — Florida’s Sen. Marco Rubio.

“There are very few candidates that have such a broad Republican appeal and a chance to reach voters that Republicans are having a hard time” winning over, said Florida Atlantic University political science professor Kevin Wagner.

Mr. Rubio, 40, has served in the Senate for less than eight months and the 2012 Republican National Convention is a year away. But for now the dapper, articulate and ambitious Miami native — who is admired by tea party activists and the party’s establishment alike — is the sexy choice for the GOP’s vice presidential candidate...