INDIAN WELLS, Calif. — Top Koch network officials on Monday warned that President Trump's polarizing leadership could turn the midterm elections into a bloodbath for the Republican Party as they pitched donors to contribute to a nearly $400 million campaign to hold back the feared Democratic wave.

Tim Phillips and Emily Seidel, of Americans for Prosperity, the Koch network's primary political organization, predicted that 80 House seats would be in play this fall and said Republicans were facing an unprecedented threat to their power — in Washington and state legislatures across the country.

To stem the tide, they implored donors gathered for the network's annual winter policy and political conference to open their wallets and fund an advertising and grassroots strategy already underway.

"Let's be frank right up front: These elections are going to be brutally tough," Seidel, the CEO of AFP, said. "Making the environment even more challenging this year are the high unfavorable ratings plaguing both President Trump and the Republican-led Congress. And the recent elections in Virginia and Alabama suggest that both anger and enthusiasm are high amongst progressive groups."

In November, Republicans unexpectedly lost a slew of seats in the Virginia legislature, as suburban voters turned against the party. In December, the Democrats captured a Senate seat in Alabama for the first time in decades. This month, Republicans lost a historically Republican state Senate seat in Wisconsin that voted solidly for Trump in 2016.

The Koch network of political and policy organizations, overseen by industrialist billionaire brothers Charles and David Koch, plans to invest close to $400 million in the fall elections, 60 percent more than it has spent in any previous election and more than it expended in the 2014 midterms or the 2016 presidential election.

Of that, around $20 million will be directed toward boosting the image of the federal tax overhaul signed into law last month that continues to meet voter resistance in electoral battlegrounds. Also within that nearly $400 million, a "multimillion dollar" investment by Concerned Veteran for America, the Koch group focused on veterans, to push veterans issues and highlight Democratic incumbents opposed to the organization's agenda.

In packed resort ballroom in an upscale desert community near Palm Springs, Calif., Phillips, AFP’s president, detailed the group’s plans to help Republicans defend majorities in Washington and in the states and the conservative policy gains they enabled.

He said the group would concentrate its paid advertising in the first six months of the year, beginning immediately, shifting emphasis to grassroots engagement for the final four months of the campaign. AFP is on the ground in 36 states and plans to play in most targeted Senate races and gubernatorial contests, with an eye toward protecting the GOP’s redistricting edge.

Phillips understands wave elections and toxic environments. He helped direct AFP’s multimillion-dollar efforts to boost Republicans in 2010 and 2014, when they won control of Congress in massive sweeps in a double backlash against President Barack Obama. House Republicans, defending a 24-seat majority, have been plagued by retirements, a development fueling Phillips’ concern.

“One last number I’ll add: 80. That’s the number of seats we believe that will be competitive in the end, this fall. Eighty,” Phillips said. “Math’s not my strong suit, but these are daunting numbers.”

On Monday, the number of Republican retirements climbed as House Appropriations Chairman Rodney Frelinghuysen, R-N.J., announced his departure, leaving yet another competitive open seat in his wake. The atmosphere in Senate races could be somewhat less challenging for Republicans, who benefit from a map of seats up for election that are friendlier to conservatives.

Still, Seidel cautioned: “This Senate majority may come down to a single seat.” Republicans are defending a razor-thin 51-49 majority, and defending vulnerable seats in Arizona and Nevada.

Koch network participants were initially disinclined toward Trump when he ran for president, put off by his course behavior and worried about the protectionist and isolationist tendencies he expressed during the campaign.

A year into Trump’s presidency, as between 500 and 600 network donors gathered in the Southern California desert 150 miles east of Los Angeles for seminars, they celebrated the conservative policy gains under Trump, notably the tax reform signed into law in December and the confirmation of Supreme Court Justice Neil Gorsuch.

Yet many also expressed lingering dissatisfaction with Trump’s demeanor and fears it would lead to steep Republican losses in Congress and in the states in November.

“A lot of people don’t like some of his tweets — the way he talks. That isn’t a secret, and that’s what I feel like. I don’t think that helps him very much,” Gary Lynch, a donor from Iowa, told reporters during a Sunday evening reception.