ABC News’ Teddy Davis reports: Florida Gov. Charlie Crist (R) will announce plans to run for the U.S. Senate on Tuesday, according to sources familiar with his plans. Crist’s entry into the race is a recruiting coup for the National Republican Senatorial Committee. The NRSC’s chairman, Sen. John Cornyn, R-Texas, has actively courted Crist to run for the seat currently being held by retiring Republican Sen. Mel Martinez. Although national Republicans have been lobbying the Florida governor to get into the race, he will not have a free ride in the primary. Crist, a moderate who publicly embraced President Obama’s stimulus package, is expected to face a conservative primary challenge from Marco Rubio, a 37-year old Cuban-American who is the former Speaker of the Florida House of Representatives. As the conservative Weekly Standard recently pointed out, an August 2010 Republican primary between Crist and Rubio could easily become "the most prominent conservative-versus-moderate Republican primary campaign in the country." Look for Rubio, the author of "100 Innovative Ideas for Florida’s Future," to try to get to the right of Crist not only on the stimulus but also on property taxes, environmental mandates, and Roe v. Wade. The conservative Club for Growth has not yet made an endorsement in the race, but the group’s president, former Rep. Chris Chocola, R-Ind., recently spoke favorably of Rubio and took a shot at Crist. “(Rubio’s) fiscally responsible, pro-growth approach in the State Capitol stands in stark contrast with other elements of the state government, led by Charlie Crist” said Chocola in a statement which greeted Rubio’s May 5th entry into the race. For now, Crist’s camp says it is not worried about the Rubio challenge, pointing to the governor’s high-approval rating and statewide name recognition. In a Quinnipiac University poll conducted in mid-April, Crist had a 66 percent job approval rating and an overwhelming lead in preference among Republicans for their party’s nomination for the Senate: 54 percent versus 26 percent undecided and single digits for all others tested (including 8 percent for Rubio, and the same for Rep. Vern Buchanan, R-Fla.). While the Crist decision is welcome news for the NRSC, it could cause heart-burn for the Republican Governors Association. Instead of being able to count on Crist cruising to re-election, the RGA will now have to contend with the Democratic Governors Association making the Florida governorship one of its top four targets along with races in California, Nevada, and Hawaii. With Crist now running for the Senate, the Florida governor’s race will likely pit state Attorney General Bill McCollum (R) against Florida Chief Financial Officer Alex Sink (D). Looking beyond 2010, some have wondered why the ambitious Crist, who would like to run for president or vice president, would run for the Senate instead of seeking re-election to a second four-year term as governor. Much of it comes down to the fact that open U.S. Senate seats do not come around all that often as well as the expiration date on Crist’s current job. Florida prohibits governors from serving more than two consecutive four-year terms. The U.S. Senate, by contrast, has no term limits which means that if he can get past Rubio and a Democratic field which includes Rep. Kendrick Meek, D-Fla., the perpetually tan Crist could be a Washington fixture for years to come. ABC News’ Gary Langer and David Chalian contributed to this report.