MADISON, Wis. (AP) — Republican Senate candidate Kevin Nicholson raised twice as much as his GOP challenger, Leah Vukmir, in the final three months of last year, based on amounts both campaigns said Monday they would formally report later in the month.

Fundraising by the candidates is important, but is expected to be vastly overshadowed by the amounts spent by outside groups backing either Vukmir, a state senator, or Nicholson, a business consultant, in indirect support of their campaigns.

Outside groups have already spent nearly $3.1 million against Democratic Sen. Tammy Baldwin in the race and an additional $2.4 million in support of Nicholson, according to the Center for Responsive Politics.

The winner of the August Republican primary between Vukmir and Nicholson will face Baldwin in November.

Nicholson, a former Marine and former national college Democratic leader who later became a Republican, is running his first race for public office. Billionaire GOP mega-donor Richard Uihlein, who lives in Illinois but started Uline Corp. in Pleasant Prairie in 1980, has donated $3.5 million to a super PAC to bolster Nicholson's campaign.

The national Club for Growth also supports Nicholson, as does a PAC run by former White House strategist Steve Bannon.

On Monday, the Wisconsin Next PAC, which supports Vukmir, said it raised over $1 million in the final three months of 2017 and would have more than $1 million cash on hand. It has not yet spent on advertising in the race. Two pro-Nicholson super PACs, Club for Growth Action and Solutions for Wisconsin, said they will report having $1.1 million cash on hand.

Vukmir is a state senator who has been in office since 2003. She's been lining up support from Wisconsin Republicans, with more than 200 endorsements to date, including billionaire Wisconsin businesswoman Diane Hendricks.

Nicholson and Vukmir declined to release details about their fundraising, including how much cash they had on hand and how much of the money came from Wisconsin residents. Nicholson's spokesman, Brandon Moody, said Nicholson did not put any of his own money in the race last quarter.

Nicholson has raised about $1.2 million since officially entering the race in late July. Vukmir, who entered in late September, has raised about $645,000.

Baldwin has yet to report her fourth quarter fundraising numbers, but she had $5.3 million cash on hand at the end of September.