Florida Sen. Marco Rubio (R) has made up his mind about running for president in 2012: He won't.

"I'm not running for president in 2012," Rubio said in an interview that will be broadcast Tuesday night on ABC's "Nightline." "Because I want to be a United States senator, I want to be the best United States senator that Florida's ever had."

Widely considered one of the GOP's rising conservative stars, Rubio's name has come up on presidential or, more often, vice presidential shortlists for 2012. While Rubio says he won't be running for president in 2012, he didn't strike down the possibility of accepting an offer to join the 2012 ticket.

Rubio's comments come as the Republican field finally starts to take shape. Minnesota Gov. Tim Pawlenty (R) recently announced plans to form an exploratory committee and Rep. Michele Bachmann (R-Minn.) is reportedly considering the formation of an exploratory committee this summer. Meanwhile, other possible GOP contenders such as former Arkansas Gov. Mike Huckabee, Mississippi Gov. Haley Barbour, and former House Speaker Newt Gingrich are strongly hinting at a presidential run. Former Massachusetts Gov. Mitt Romney is also expected to enter the race.