Sen. Thad CochranWilliam (Thad) Thad CochranObstruction of justice watch: Trump attacks the FBI America isn't ready to let Sessions off his leash The Hill's Whip List: Where Republicans stand on Senate tax bill MORE's (R-Miss.) Tea Party challenger is off to a quick fundraising start, raising $500,000 in his first 10 weeks in the race.

Mississippi state Sen. Chris McDaniel (R), who has the backing of many conservative groups including the Club for Growth, FreedomWorks and the Senate Conservatives Fund, could be Cochran's toughest foe in decades.

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The strong haul, likely helped by those groups' national fundraising networks, shows he'll have the funds to make Cochran work hard for reelection. McDaniel has $350,000 in the bank for the race.

"All across Mississippi and all across the nation, we're seeing tremendous enthusiasm and excitement, as grassroots conservative activists are uniting behind our campaign," McDaniel said in a statement. "Conservatives are clamoring for new leaders in Washington who will fight to repeal Obamacare, rein in federal spending, and protect our traditional values. With such a groundswell of enthusiasm, I am confident we will succeed on June 3rd."

Cochran hasn't yet released his fundraising figures, but had $800,000 through the end of September. The GOP incumbent didn't declare he would run for reelection until early December, but may be gearing up fast: According to theĀ Clarion Ledger, he netted $300,000 in one fundraiser earlier this week.

This post was updated at 5:25 p.m.