The decision by the four-term lawmaker could complicate the GOP's chances of holding onto the seat as Republicans battle to retain their Senate majority in this November's elections.

Loeffler, 49, was appointed this month to the seat by Republican Gov. Brian Kemp to replace the retired Sen. Johnny Isakson, who left office due to deteriorating health. She's a businesswoman and a newcomer to politics.

The conservative Collins, top Republican on the House Judiciary Committee, was Trump's preferred choice for the appointment. Trump supporters had criticized Loeffler as too moderate.

But Kemp picked the wealthy Loeffler, who has said she will spend $20 million to retain the seat, in what was viewed as a bid to woo suburban and female voters. Since Kemp chose her, Loeffler has emphasized a pro-Trump message on issues like gun rights and building a wall along the border.

Besides Loeffler's considerable piggy bank, Collins will also face opposition from the National Republican Senatorial Committee. The Senate GOP's campaign arm traditionally supports incumbent Republican senators against primary challenges and is supporting Loeffler.

Loeffler will also be supported by the influential U.S. Chamber of Commerce, said Scott Reed, the organization's senior political strategist. He called her "exactly the type of pro-business and pro-growth candidate we support."

The GOP official who described Collins' expected plans spoke on condition of anonymity to reveal private conversations. Collins' move was reported Monday night by the Atlanta Journal-Constitution and Politico.

Another complication in the race arose Monday when a Georgia legislative panel approved a bill that could change the state's party primary system.

Under current Georgia law, Loeffler and any other candidates, regardless of party, would run together in a November special election. If no one wins a majority of the vote, a runoff would be held in January — with control of the Senate potentially in the balance.

But the new bill, opposed by Kemp, would establish May primaries. The measure was approved Monday by a subcommittee of Georgia's House Governmental Affairs Committee.

Kemp promptly issued a veto threat.

"You don't change the rules at half-time to benefit one team over another," spokeswoman Candice Broce said in a statement.

So far, two Democrats have announced plans to challenge Loeffler. Matt Lieberman is an Atlanta educator and the son of former vice presidential candidate Joe Lieberman. Ed Tarver of Augusta served as U.S. attorney for Georgia's Southern District during President Barack Obama's two terms.

The Rev. Raphael Warnock, who pastors the Atlanta church once led by Martin Luther King Jr. and his father, is considering a bid and could announce soon. He's widely seen as the favorite of party leaders, and a primary could allow the winner to consolidate party support.