Twelve Republicans have announced they will compete for the GOP nomination in 2016. The latest entry to the race is businessman and television personality Donald Trump.

Four Republican governors -- Louisiana's Bobby Jindal, Wisconsin's Scott Walker, New Jersey's Chris Christie and Ohio's John Kasich -- are considering presidential bids.

Mitt Romney, the 2012 GOP nominee, said he would not run for president a third time after considering another bid. Former U.S. Ambassador to the United Nations John Bolton also passed.

So far, former Secretary of State Hillary Clinton has three Democratic challengers: Bernie Sanders, an independent senator from Vermont, former Maryland Gov. Martin O'Malley, and former Rhode Island senator and governor -- and onetime Republican -- Lincoln Chafee.

Former Virginia Sen. Jim Webb and Vice President Joe Biden have expressed an interest in the 2016 race.

Despite being urged on by a fervent fan base, Massachusetts Sen. Elizabeth Warren, D-Massachusetts, has repeatedly said she is not running for president.