ATLANTA — A top Republican state senator threw his hat in the 2010 governor's race Monday.

Eric Johnson, of Savannah, had been running for lieutenant governor. But he swapped races after Lt. Gov. Casey Cagle - a presumed front runner for the GOP nomination for governor - withdrew from that contest earlier this month citing health problems. Mr. Cagle said he'd instead seek re-election to his current seat. That pitted him against Mr. Johnson.

Mr. Johnson made the announcement Monday as Mr. Cagle was undergoing surgery for a degenerative spinal condition.

Mr. Johnson said his campaign would focus on jobs, education and ethics.

He joins an already crowded field to replace Gov. Sonny Perdue when his second term expires. Among Republicans, Secretary of State Karen Handel, Insurance Commissioner John Oxendine and state Rep. Austin Scott have said they are running.

On the Democratic side, Attorney General Thurbert Baker, House Minority Leader DuBose Porter and former Georgia National Guard Commander David Poythress are in. Former Gov. Roy Barnes will decide whether to run by June 1.

Mr. Johnson is the former president pro tem of the Senate. He gave up the post this year to run for lieutenant governor. Mr. Johnson pushed unsuccessfully to make Georgia the first state in the nation to offer universal school vouchers. That would have expanded on a program he ushered through in 2007 that grants vouchers to special needs students.

He is chairman of the Senate Ethics Committee and led the effort this year to crack down on legislators who haven't paid their taxes.