LOS ANGELES -- A slate of Republican U.S. House members in heavily Democratic California is facing primary challengers on Tuesday in races that will help determine control of Congress.

No incumbents appear at risk of losing their primary, but the districts will be among the country's marquee races in November. Two of the House members are trying to surmount challenges tied to former U.S. President Donald Trump: One voted to support the former president's impeachment after the U.S. Capitol insurrection, while the other fought against it.

Elsewhere on Tuesday, former Interior Secretary Ryan Zinke is competing against several other Republicans for a chance to capture a new U.S. House district in western Montana. In Iowa, a trio of Republicans is jockeying to take on the state's lone Democratic U.S. House member in a newly drawn district with a stronger GOP tilt. And in Mississippi, Republican U.S. Rep. Steven Palazzo is running for reelection after a congressional ethics watchdog raised questions about his campaign spending.

A look at key U.S. House races:

TRUMP FACTORS INTO GOP RACES IN HEAVILY DEMOCRATIC CALIFORNIA

Two Republican congressmen are facing challenges tied to Trump, but for different reasons.

In a Democratic-tilting district in the state's Central Valley farm belt, Republican Rep. David Valadao is highlighting an independent streak while contending with GOP fallout for his vote to impeach Trump over the Jan. 6 insurrection. Republican Chris Mathys has made Valadao's vote a centerpiece in his campaign to oust him.

In a Democratic-leaning district north of Los Angeles, three Democrats are vying for the chance to take on Republican Rep. Mike Garcia, who captured the seat in 2020. Garcia, a former Navy fighter pilot who was endorsed by Trump in 2020, joined House Republicans who rejected electoral votes from Arizona and Pennsylvania and opposed Trump's impeachment after the Capitol insurrection. Two other Republicans are also on the ballot.

California uses a top-two election format in which only the top two vote-getters advance to the November general election, regardless of party.

FORMER TRUMP CABINET MEMBER SEEKS RETURN TRIP TO WASHINGTON

Montana gained a second congressional district this year thanks to its growing population, and Zinke, an Interior Department secretary under Trump, is one of five Republicans on the primary ballot for the open seat.

Zinke's rivals have been drawing attention to his troubled tenure at the agency, which was marked by multiple ethics investigations. One investigation determined Zinke lied to an agency ethics official about his continued involvement in a commercial real estate deal in his hometown. He's faced a smear campaign over his military service from the extreme right wing of his party and questions about his residency following revelations that his wife declared a house in California as her primary residence.

Zinke, a former Navy SEAL, is widely considered the de facto incumbent, since he twice won elections for the state's other House seat before stepping down in 2017 to join the Trump administration.

His primary opponents include former state Sen. Al "Doc" Olszewski, an orthopedic surgeon and hard-line conservative who has tried to paint Zinke as a "liberal insider." Three Democrats are vying for their party's nomination: public health advocate Cora Neumann, Olympic rower and attorney Monica Tranel and former state Rep. Tom Winter.

IOWA'S SOLE DEMOCRATIC HOUSE MEMBER FACES A TOUGH FIGHT

Three Republicans are competing for a chance to run against two-term Democratic Rep. Cindy Axne in a newly drawn Iowa district that could be more favorable for the GOP.

In previous elections, Axne was elevated by her strong support in the Des Moines area, even as she struggled in rural counties that typically lean Republican. The new district includes several counties in southern Iowa known to turn out strongly for Republicans, increasing the pressure on Axne to drive up her numbers in Democratic-friendly Des Moines and its suburbs.

Among the Republicans, state Sen. Zach Nunn is the best known of her challengers. He's an Air Force pilot who has served in the Legislature since 2014 and has worked to cut taxes. He's running against Nicole Hasso, a financial services worker, and Gary Leffler, who works in the construction industry.

MISSISSIPPI CONGRESSMAN WITH ETHICS TROUBLE DRAWS A CROWD

First elected in 2010, Republican Rep. Steven Palazzo of Mississippi is facing his largest-ever primary field after a congressional ethics watchdog raised questions about his campaign spending.

A 2021 report by the Office of Congressional Ethics found "substantial reason to believe" Palazzo, a military veteran who serves on the Appropriations and Homeland Security committees, abused his office by misspending campaign funds, doing favors for his brother and enlisting staff for political and personal errands. His then-spokesperson, Colleen Kennedy, said the probe was based on politically motivated "false allegations."

His six opponents include a sheriff, Mike Ezell, and a state senator, Brice Wiggins. If no candidate wins a majority of votes, a runoff will be June 28.

A REPUBLICAN MATCHUP FOR SOUTH DAKOTA'S ONLY HOUSE SEAT

Trump also is playing into the fight for South Dakota's lone House seat.

GOP Rep. Dusty Johnson, a member of the bipartisan Problem Solvers Caucus, is facing a challenge from his political right from Republican state lawmaker Taffy Howard. She has echoed Trump's discredited claims of widespread voter fraud and criticized Johnson for voting to certify the results of the 2020 presidential election.

Johnson, however, has maintained a wide fundraising edge. No Democrats are running, so the winner of the GOP primary will likely take the seat.

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Associated Press writers Scott McFetridge in Des Moines, Iowa; Emily Wagster Pettus in Jackson, Miss.; Matthew Brown in Billings, Mont.; and Stephen Groves in Sioux Falls, S.D., contributed to this report