Former US Republican presidential candidate Marco Rubio says he won't be anybody's running-mate in the November 8 presidential election, or seek any public office when his term in the US Senate ends.

"No, I'm not going to be anybody's vice president," Mr Rubio told reporters on Capitol Hill.

"I'm going to finish out my term in the Senate over the next 10 months ... and then I'll be a private citizen in January."

Mr Rubio, a US senator from Florida, dropped out of the race for his party's nomination following a loss in his home-state of Florida to party front-runner and political upstart Donald Trump on Tuesday.

His decision to quit leaves the Republican nomination a three-horse race between Mr Trump, Ted Cruz and John Kasich.

Despite Mr Trump's lead, Mr Rubio is confident either Mr Cruz or Mr Kasich will come from behind to defeat him.

"I think that's going to happen now… hopefully there's time to still prevent a Trump nomination which I think will fracture the party."

Mr Rubio also ruled out running for another position, such as Governor.

Reuters / Newshub.