McBath's move opens the door for another up-and-comer to enter the race: Jen Jordan. The Sandy Springs Democrat has emerged as a force in Georgia politics since flipping a state Senate seat in a 2017 special election. She's one of the sharpest critics of Georgia's new anti-abortion laws, and has led the charge to close down a plant in her district that's leaking cancer-causing gasses.

Other Democratic candidates whose names have been floated include state Sen. Nikema Williams, chairwoman of the Democratic Party of Georgia; DeKalb County District Attorney Sherry Boston; DeKalb County Chief Executive Michael Thurmond; and Rev. Raphael Warnock, the senior pastor of the Rev. Martin Luther King Jr.'s historic congregation.

Gov. Brian Kemp recently issued the political equivalent of an open casting call to help find a Republican to replace Isakson after he steps down on Dec. 31. His pick will likely be on the ballot again in November 2020 to fill the remainder of Isakson's term.

More than 150 candidates have already applied for the position, with many more expected to join in. Several GOP members of the state’s congressional delegation have expressed interest, including Doug Collins and Tom Graves.

Other potential Republican candidates include Lt. Gov Geoff Duncan, state Attorney General Chris Carr and Congressman Drew Ferguson.

Republicans were quick to characterize McBath’s decision as a recruiting failure for Democrats.

"With both Stacey Abrams and Lucy McBath refusing to run for Senate, Georgia Democrats have now struck out with both of their top two possible contenders for Georgia's Senate seats,” said John Burke, executive director of the GOP super PAC the Georgia Action Fund. “Control of the U.S. Senate will likely come down to this state and Democrats remain without a single candidate who can win in 2020."

A Senate run hadn't been on McBath's radar until Isakson's sudden retirement announcement prompted a blitz of calls from state and national operatives trying to recruit her. She'd also briefly left open the possibility of challenging incumbent Republican U.S. Sen. David Perdue, who is up for re-election in 2020.

McBath became a household name last year after her surprise ouster of Karen Handel in Georgia's 6th District, which includes portions of Cobb, Fulton and DeKalb counties.

A gun control advocate before coming to Congress – her teenage son was fatally shot in 2012 – McBath has become one of House Democrats' most visible voices on the issue in recent months. She helped shepherd legislation to expand background checks to passage earlier this year, and in recent weeks she's upped the pressure on Senate Republicans to move the bill, along with a federal "red flag" measure to keep firearms out of the hands of people declared dangerous to themselves and others.

She's being challenged for re-election by a quartet of Republicans, including Handel and state Sen. Brandon Beach.

Read more: Online 'help wanted' sign sets off Senate scramble in Georgia