A voter casts her ballot at a Los Angeles County lifeguard headquarters at Venice Beach on June 5, 2018 in Los Angeles, California.

A slate of primary elections in eight states Tuesday will have perhaps the biggest stakes yet this year in the battle for Congress.

The most populous American state, California, headlines the day's primaries. Democrats are targeting up to 10 Republican-held House seats there, and could flip a significant portion of the 23 GOP seats they need in November's midterms to take control of the House. But an abundance of Democratic candidates running in the state's top-two primary system — which allows the two highest vote-getters regardless of party affiliation to move forward — could lead to the Democrats getting shut out of one or more key swing district general elections.

President Donald Trump stressed the importance of the state's House races in a tweet Tuesday morning urging voters to support Republican candidates. He and House Republican leaders have frequently used California's most prominent Democrat, House Minority Leader Nancy Pelosi, as a midterm attack target against her party.

"Keep our country out of the hands of High Tax, High Crime Nancy Pelosi," the president wrote.

Trump tweet

Voters will also choose nominees for critical House races in states such as New Jersey and Iowa. Democrats have a chance to pick up several GOP-held seats in New Jersey.

Meanwhile, voters in red Montana will choose a nominee to face Sen. Jon Tester, one of the chamber's most vulnerable Democrats. Five of the states, including California, will start the process of electing governors, as well.

Here are some of the races to watch: