When President Donald Trump threw his endorsement Friday to John Cox, a little-known businessman running for California governor, he also gave House Republicans a potentially significant lift in their effort to maintain control of Congress.

Republicans in this heavily Democratic state long ago abandoned almost any hope of winning the governorship, but getting shut out in California’s primary — where the top two finishers advance, regardless of party — could depress turnout and lead to devastating losses in House races statewide. In a warning to Republicans in April, former House Speaker Newt Gingrich wrote that Cox advancing to the November ballot “will be vital for keeping the Republican majority.”

Trump’s endorsement of Cox, a wealthy businessman who previously ran unsuccessfully for House and Senate seats in Illinois and, briefly, for president in 2008, was the culmination of weeks of effort by Republican leadership, including endorsements from House Majority Leader Kevin McCarthy and Rep. Devin Nunes (R-Calif.). The president’s intervention is considered especially significant as Cox failed to secure the California Republican Party’s endorsement, splitting activists’ support at the state party’s convention this month with another Republican, state Assemblyman Travis Allen.