GOP Rep. Jeff FlakeJeffrey (Jeff) Lane FlakeAuthorizing military force is necessary, but insufficient Republicans jockey for position on immigration McCain, Flake warn against 'politically-motivated penalties' for Canadian defense firm MORE (Ariz.) will announce Monday that he is running for the Senate in 2012.

According to a Republican source familiar with his plans, Flake will hold an 8 a.m. Mountain Time news conference in Phoenix to declare his candidacy to replace Sen. Jon Kyl (R-Ariz.), who is retiring. The seventh-term congressman has long been rumored to want a spot in the upper chamber.

Flake appears to be the candidate of choice among national Republicans for the open-seat race. Right after Kyl announced his retirement last week, GOP insiders pegged Flake as the early front-runner, despite the potential for a contested primary. 

He's also a favorite of the Club for Growth, which has already publicly urged him to jump into the race.   

Flake could face a crowded GOP field. A number of Republicans, including Rep. Trent FranksTrent FranksOvernight Health Care: House passes 20-week abortion ban | GOP gives ground over ObamaCare fix | Price exit sets off speculation over replacement House passes 20-week abortion ban Trump administration backs 20-week abortion ban MORE (Ariz.) and former Rep. J.D. Hayworth (Ariz.), have said they are also considering running. 

Democrats are also targeting Kyl's seat as a pickup opportunity, and some party members in Arizona are holding out hope for the recovery of Rep. Gabrielle Giffords (D-Ariz.), who, before she was shot at a public event in January, told staff she would run for Senate if Kyl retired.

This post was updated at 7:40 a.m.