Conservative billionaire Charles Koch and his family have maxed out to Iowa state Senate candidate Joni Ernst (R), a sign of the Koch brothers' particular interest in helping her campaign.

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Koch, his wife, his son and daughter-in-law each gave the legal maximum contribution of $2,600 to Ernst in late June, according to newly filed documents with the Federal Election Commission. Koch Industries also made a $5,000 contribution to her campaign.

She's just the second candidate all four Kochs have maxed out to this year. Sen. Tim ScottTimothy (Tim) Eugene ScottSunk judicial pick spills over into Supreme Court fight Controversial Trump judicial nominee withdraws GOP senators introduce resolution endorsing ICE MORE (R-S.C.) is the other.

The donations aren't a shock. Americans for Prosperity, the big-spending conservative group partly bankrolled by Charles and his brother David, has already been on the air in Iowa pounding Ernst's opponent, Rep. Bruce BraleyBruce Lowell BraleyOPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer MORE (D-Iowa).

The race for retiring Sen. Tom HarkinThomas (Tom) Richard HarkinDem Senator open to bid from the left in 2020 Senate GOP rejects Trump’s call to go big on gun legislation Trump should require federal contractors to follow the law MORE's (D-Iowa) seat has emerged as a marquee Senate battle, and recent polls show a close fight.

Democrats have been looking to tie Ernst to the billionaires and their financial interests, but her campaign pointed fingers back at Braley's donors, too. 

"The real story for Iowans are the countless liberal extremists bankrolling Bruce Braley's campaign," said Ernst spokesman Derek Flowers. 

—This post was updated at 5:32 p.m.