Super down-ballot Tuesday: Jeff Sessions, George Papadopoulos and other key congressional primaries to watch

While not getting as much attention as the headliner race, primaries in California, Texas, North Carolina and Alabama could vault unknown names into key matchups -- or stop some well-known political figures in their tracks.

Here's a few key matchups to keep an eye on when polls close Tuesday evening:

Alabama - Republican primary for US Senate

Republicans want to reclaim the Senate seat in deeply red Alabama held by Democrat Doug Jones, but there's a crowded primary of GOP candidates, meaning the race is all but assured to head to a runoff at the end of the month. Among those Republican hopefuls: former attorney general Jeff Sessions, former Auburn football coach Tommy Tuberville, US Rep. Bradley Byrne and former Alabama Supreme Court Justice Roy Moore, who lost to Jones in a special election more than two years ago. CNN's Alex Rogers has a thorough rundown of that race and explains why all the candidates are trying to prove they're the candidate closest to President Donald Trump.

Most polls close at 8 p.m. ET.

North Carolina - Democratic primary for US Senate

Republican Sen. Thom Tillis is up for reelection in this state that has proven to be competitive in the past, but the Democratic primary for picking his challenger has seen an unexpected influence. A group aligned with Senate Majority Leader Mitch McConnell has funneled nearly $3 million into ads in the race backing a liberal candidate. Why? Insurgent Democratic state Sen. Erica Smith is challenging former state Sen. Cal Cunningham, who won the endorsement of the Democratic Senatorial Campaign Committee. CNN's Lauren Fox and Ellie Kaufman explain why GOP-aligned groups are boosting Smith in hopes of giving Tillis a weaker general election opponent in the fall.

Polls close at 7:30 p.m. ET.

Texas - Primaries for US House seats

If you were looking for an upset win Tuesday, the Lone Star State's US House race primaries might be a good place to start. Texas' large and diverse population puts its 36 congressional races at the forefront of America's changing demographics and political landscape. One example: incumbent Rep. Henry Cuellar, one of the most conservative Democrats in the chamber, is facing a challenge from Jennifer Cisneros, Cuellar's former intern, who's backed by New York. Rep. Alexandria Ocasio-Cortez.

Another possible upset worth watching: 12-term Rep. Kay Granger, the top Republican on the Appropriations Committee, is facing a primary challenger backed by the anti-tax Club for Growth. Granger, one of just 13 GOP women in the House, has Trump's endorsement, but should she lose, House Republicans would be down to just 10 women running for reelection this fall.

Also in Texas, look for some familiar faces hoping to return to Washington, including Trump's former chief physician, Rear Adm. Ronny Jackson, who is running for the GOP nomination in Texas' 13th congressional district.

Polls close at 8 p.m. ET and 9 p.m. ET.

California - Primaries for US House seats

The nation's most populous state also goes to the polls Tuesday, and California's House races could feature some additional down-ballot drama.

One vacated seat -- formerly held by Democratic Rep. Katie Hill, who resigned last year -- is in a very competitive district, and 12 candidates are vying for the spot. Among the colorful candidates are George Papadopoulos, "Young Turks" talk show host Cenk Uygur and the Republican who held the seat before he was ousted in 2018. CNN's Clare Foran breaks down the complicated mechanics of this special election within primary day.

Elsewhere in the state, former Rep. Darrell Issa, a well-known Republican and former committee chairman, is running in a contested primary for the seat vacated by GOP Rep. Duncan Hunter, who resigned from Congress earlier this year, in a district Democrats see as competitive.

Polls close at 11 p.m. ET.