Washington (CNN) President Donald Trump tweeted support for Republicans running in a slate of primaries across the country on Tuesday, including some his key GOP allies on Capitol Hill.

Trump urged voters to back several congressional incumbent Republicans: House Majority Leader Kevin McCarthy; Republican Rep. Devin Nunes, the chairman of the House Intelligence Committee; and Republican Sen. Roger Wicker of Mississippi

He also called on voters to support Republican John Cox for governor in California, a state he called "High Tax, High Crime California."

Political observers are closely watching races in California Tuesday night, where Democrats hope several key races could later play a role in switching the balance of power in the House.

Nunes is running in one of the those races, aiming to hold onto his seat representing California's 22nd District. Fresno County Deputy District Attorney Andrew Janz, a Democrat, is expected to finish in second place behind Nunes in the state's unusual primary system, where the top two contenders in a primary race -- regardless of their party affiliation -- move on to the general election. Trump won the district with 52%, and while Janz has raised quite a bit of money, the district is still considered safe for Republicans in November.

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