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 ScottNew signs of progress emerge on police reform White House to give Congress space on police reform The Hill's Morning Report - Presented by Facebook - After historic verdict, Chauvin led away in handcuffs 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 BraleyThe Memo: Trump attacks on Harris risk backfiring 2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP MORE (D-Iowa).

The race for retiring Sen. Tom HarkinThomas (Tom) Richard HarkinWe need a voting rights workaround Romney's TRUST Act is a Trojan Horse to cut seniors' benefits Two more parting shots from Trump aimed squarely at disabled workers 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.