The head of a Washington committee that helps Republicans get elected says Florida Gov. Charlie Crist should drop out of the Senate race if he doesn't think he can win the GOP primary.

National Republican Senatorial Committee executive director Rob Jesmer wrote in an e-mail Monday that he believes there is "zero chance" Crist will run as a Republican.

He says Crist should drop out of the race if he believes he can't win the primary instead of running as an independent candidate. He says the committee is prepared to support Crist's primary opponent, former House Speaker Marco Rubio, if Crist runs as an independent.

Former Massachusetts Gov. Mitt Romney campaigned with Rubio in Tampa on Monday and said Crist shouldn't run as an independent.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

TAMPA, Fla. (AP) — Former Massachusetts Gov. Mitt Romney challenged Florida Gov. Charlie Crist on Monday to either stay in the U.S. Senate race as a Republican or step aside and support his opponent.

Romney's comments came at a Tampa rally where he officially endorsed challenger Marco Rubio. There is speculation that Crist might bolt the Republican Party and run as an independent, but Romney said he expects Crist to "do the right thing."

Romney is often mentioned as a potential 2012 presidential candidate. By backing Rubio, a former state House speaker and a favorite of conservatives, he could help endear himself to Republican voters who were uncomfortable with his moderate stands on abortion and gay rights when he ran for office in Massachusetts.

Crist's campaign did not immediately respond to an e-mail and phone messages seeking comment Monday.

Recent polls show Crist is well behind Rubio in the Aug. 24 Republican primary but could win a three-way general election race with Rubio and the likely Democratic nominee, U.S. Rep. Kendrick Meek.

In an interview after the event, Rubio declined to issue the same challenge as Romney, saying the reason he is running will not change no matter who is in the race.

"I think we need to send someone to Washington who will stand up to the Obama agenda and offer an alternative, and both Kendrick Meek and Charlie Crist, I think, to one extent or another, would support the Obama agenda," Rubio said. He said he hadn't thought about how his campaign strategy would change with Crist running as an independent.

The governor must decide by April 30 if he will remain in the Republican primary or take his chances in the general election without a party. He cannot switch after the primary.

Crist endorsed Sen. John McCain over Romney just days ahead of Florida's 2008 GOP presidential primary, and some political observers believe that was enough to give McCain the win, which he used to build momentum and capture the nomination.

Two other 2008 presidential candidates — former New York Mayor Rudy Giuliani and former Arkansas Gov. Mike Huckabee — have endorsed Rubio too. Giuliani was also counting on a Florida victory during the 2008 campaign and now says Crist backed out of a promised endorsement.

Romney said Monday that he respects Crist, but Rubio's "proven record of conservative, principled and idea-driven leadership is what Florida needs now."

Romney's PAC is contributing the maximum $5,000 allowed to Rubio's campaign. In 2006, Romney flew to Tallahassee as head of the Republican Governors Association with a $1 million check to help Crist's run for governor.

Crist, Rubio and Meek are running to fill the seat held by George LeMieux, Crist's former chief of staff. Crist appointed him after Mel Martinez resigned before completing his first term, saying he wanted to spend more time with his family.