Loeffler, a political newcomer little known to many local GOP officials, has aggressively tried to win over the president - and prevent a Collins run. She has slammed the "impeachment circus" and promised to support his agenda.

And on Monday, Loeffler sent a scathing tweet accusing fellow Republican U.S. Sen. Mitt Romney of trying to "appease the left" over a vote on witnesses at the impeachment trial, and then announced she hired a former Trump aide to run her campaign.

But it didn't ward off Collins, who said even before Loeffler was appointed that he would "strongly" consider challenging her. He declined to comment late Monday.

No primary

With Collins' decision, Kemp's hopes of unifying Republicans behind his Senate appointment are dashed at a time when the GOP can't afford much discord. Democrats have circled Georgia as a top 2020 target, eager to flip Georgia's two Senate seats and upend the GOP's 53-47 control of the chamber.

Unlike traditional elections that have primaries to hash out Democratic and Republican nominees, the race to fill the remaining two years of retired U.S. Sen. Johnny Isakson's term is a wide-open special election that features multiple candidates from all parties on the same ballot.

A Georgia House subcommittee voted earlier Monday to create a partisan primary to force Loeffler and Collins in a head-to-head matchup that would favor the veteran lawmaker, but Kemp has threatened to veto the legislation.

The prospect of multiple well-funded Republicans slicing into each other’s base on a November ballot raises the likelihood of a January 2021 runoff if no candidate gets a majority of the vote – and the possibility that a Democrat who consolidates party support can win the contest outright.

Georgia Gov. Brian Kemp greets President Donald Trump at Dobbins Air Force Base on Nov. 8, 2019, in Marietta. (Curtis Compton/ccompton@ajc.com)

Top Georgia Democrats, too, talk of unifying behind one candidate who is expected to soon enter the race: the Rev. Raphael Warnock, the pastor of Ebenezer Baptist Church. But several other Democrats plan to qualify, including Matt Lieberman, an entrepreneur, and former federal prosecutor Ed Tarver.

At the same time, three well-financed Democrats are taking aim at U.S. Sen. David Perdue, who is also on the November ballot. The former Fortune 500 chief executive is seeking his second -- and what he says final -- term in office.

'Fighter’

Collins will likely center his campaign on his support from grassroots activists and allegiance to Trump. An Atlanta Journal-Constitution poll released last week showed more than 90% of Republicans approved of the president's job performance.

He’s helped by his lead role on the House Judiciary Committee, which made him a constant presence on cable TV to lob broadsides against the Democratic-led impeachment of Trump, and his mountainous north Georgia district is home to one of the most conservative electorates on the Eastern Seaboard.

He's also forged deep ties with the president's inner circle. Donald Trump Jr. recently headlined a high-dollar fundraiser for Collins, calling him the type of "fighter" Republicans need in the Senate, and Collins counts Jared Kushner, Trump's son-in-law, as an adviser. Fox News personality Sean Hannity regularly refers to the Georgia congressman as "Senator Collins."

But in Loeffler, Collins will face no easy challenger.

She has the full-throated support of Kemp, who was the most popular politician in Georgia in the AJC poll. She'll have the backing of outside groups, including the National Republican Senatorial Committee. And she's pledged to spend at least $20 million of her own cash to finance the race.

Mindful of a potential challenge from the party’s right flank, Loeffler has raced to shore up her conservative credentials - and prove herself to Trump - since she took office this month.

She's debuted a pro-Trump television ad, hired veteran conservatives to staff her campaign and fired off tweets bashing of impeachment.

Both Loeffler and Collins have plenty of work ahead building up their name recognition. The AJC poll found that only about a quarter of registered voters have a favorable impression of Loeffler, and that roughly 60% of fellow Republicans don't know enough about her to answer.

Collins, too, was unknown to roughly half the electorate. But he had a far higher profile with Republicans: About 53% of Republican voters had a favorable impression of him, compared to just 6% who have negative views.