As president, Donald Trump has insulted US allies and top Republicans, threatened nuclear war on Twitter, shared top-secret intelligence with Russia, and pushed immigration policies that are ripping apart families and damaging the US’s image abroad.

Until now, though, the Republican Party stood by its president. The majority of Republican senators and representatives have confirmed Trump’s nominees, agreed to blow up the US deficit, and generally declined to comment on unsavory details about his personal life. Trump’s approval ratings have also remained strong among Republican voters. As former speaker John Boehner recently said, “There is no Republican Party. There’s a Trump party.”

But last week, Trump went too far.

On May 31, in the name of national security, the president passed steel and aluminum tariffs that threaten trade with the US’s closest allies. The announcement drew furious criticism from members of his party, because it goes against their pro-business agenda. Now some of the biggest Republican donors have launched a war against Trump’s tariffs, and once-loyal senators are trying to curb Trump’s powers as the country’s 45th president.

A bill that kills Trump’s trade tariffs

This afternoon, a bipartisan group of senators introduced a new bill that takes away Trump’s right to pass tariffs on products that the Department of Commerce deem important to national security. The bill would require any intended tariffs to be approved by Congress. It would also be retroactive for the past two years—meaning it could rescind last month’s controversial steel and aluminum tariffs.

The bill is sponsored by Republicans and frequent Trump critics Jeff Flake of Arizona and Bob Corker of Tennessee. It also includes four other GOP senators (and four Democrats). The tariffs will be bad for US business, and US jobs, they say, echoing recent analysis by economists.

Imposing tariffs “under the false pretense of ‘national security’ weakens our economy, our credibility with other nations, and invites retaliation,” said Pat Toomey, the Pennsylvania Republican who voted with Trump 91.4% of the time.

“These kinds of tariffs are a big mistake, and using national security as an excuse is a bigger mistake,” said Lamar Alexander, the Republican from Tennessee, who has voted in support of Trump more than 94% of the time in the past.” Ron Johnson, a Wisconsin Republican who has voted with Trump nearly 92% of the time, said Congress should assert “its constitutional authorities on tariffs to ensure we don’t undermine the significant economic progress we have made over the last 18 months.”

If senate majority leader Mitch McConnell calls a vote on the bill in coming days, it will be a sign that he, too, has decided it is time to curb Trump.

The Kochs take away their checkbook

Meanwhile, the deep-pocketed Koch brothers, Republican kingmakers, went on the offensive against Trump earlier this week, rolling out a new, and radical list of trade-friendly demands on June 4. They plan to push it through their think-tank, grassroots political network, and Hispanic libertarian group.

The demands are aimed specifically at easing Trump’s protectionism, as well as dismantling a host of long-standing US policies, some of which have been popular with industries and voters for decades:

The president should lift steel and aluminum tariffs, drop proposed tariffs on China, and “avoid any new tariffs.”

The president should renegotiate NAFTA, and reenter talks on the Trans-Pacific Partnership.

Congress should abolish farm and green energy subsidies, and repeal the “buy American” rules on federal projects.

Congress should pass an amendment to limit the president’s ability to pass tariffs.

The Koch brothers are considered the architects of the 2010 “Republican wave” that swept Congress after Barack Obama’s election. They and their network of donors spent an estimated $1 billion on the 2016 election. They initially refused to back Trump, but since he won, they’ve wooed the president and been rewarded: His administration has already adopted dozens of their regulation-gutting, tax-rescinding ideas.

Now the Kochs are taking their checkbook, influential foundation, and activists, away. Anyone who doesn’t back their free trade principles is “going to have a harder time getting support from our network,” an official with the Koch network told Quartz. That means the Kochs also are unlikely to support Congress members in heated 2018 midterm races who back Trump’s trade stance.

(On June 5, Charles Koch said his brother David would retire from political and business activities due to ill health. His departure doesn’t change the Koch network’s new campaign, the official said.)

This new direction could even mean funding Democrats sometimes, the Koch official said. Politicians who support free-trade principles, regardless of party, “are people we’re going to want to work with and work with more often.”

The siphoning away of financial support could be disastrous for Trump; if Democrats take the House in November, there’s a much greater chance he could be impeached.