The stage was set months ago for some unusual Congressional contests in the California primary election when voters overwhelmingly passed Prop 50.

On Election Day Tuesday, the redrawn Congressional maps that were approved Nov. 4 with passage of the statewide ballot measure will get a first test. The maps, spearheaded by Democratic Gov. Gavin Newsom, were intended to tip as many as five Republican-held seats to Democrats this year to offset President Donald Trump’s push in Texas to gain five seats for his party there.

A series of similar efforts in other states followed.

A key element in how the gerrymandering plays out is California’s open primary system itself — the top two vote-getters in the primary election advance to the November runoff. That structure set up some closely watched matchups in Southern California.

Here’s what to know and some key House races to watch.

Congressional Districts 40: The battle of incumbent Republicans

In Southern California, sitting Republican Reps. Ken Calvert and Young Kim were drawn into the same conservative district and are battling over their pro-Trump credentials, which have been disputed in dueling TV ads on heavy rotation.

The race is one of the Prop 50 maps’ biggest alterations. Calvert, R-Corona, has been in office since the 1990s. His old 41st district was pushed north, and he is now competing directly with Kim, R-Anaheim.

The two incumbents are now battling in the newly defined 40th congressional district.

Along with Calvert and Kim, six other candidates are in the running, though the two incumbents are favored. The bulk of the district’s metropolitan space is in Orange County, but the eastern boundary juts into Riverside County, encompassing the Temescal Valley, most of Lake Elsinore, Canyon Lake and Menifee.

Calvert’s prior stomping ground of northwest Riverside County, including Corona, Eastvale and Norco, has been lumped into the 35th congressional district, where Rep. Norma Torres, D-Ontario, is pitted against Republican businessman Mike Cargile. Most of the 35th district is in San Bernardino County and is considered a Democrat stronghold.

Congressional District 48: Riverside and San Diego counties

Long-time Rep. Darrll Issa announced his retirement in March after Prop 50 resulted in changes to his Southern California district. In a statement announcing his retirement, Issa endorsed San Diego County Supervisor Jim Desmond as his successor in the district that includes parts of San Diego and Riverside counties.

Desmond also has President Trump’s endorsement in a race that will likely have national ramifications.

Once considered firm Republican territory, the new map slightly favors Democrats — of which there are many competing in the primary. But with no clear top-two favorites in a close race, Democrats have a chance of effectively missing out on picking up a seat in the San Diego suburbs.

San Diego City Councilmember Marni von Wilpert and Ammar Campa-Najjar, a three-time congressional candidate from Chula Vista, appear to be the Democratic frontrunners. Von Wilpert had nearly 55% of delegates’ votes at the February California Democratic Party convention, just shy of the endorsement-clinching %60 threshold. Campa-Najjar was at %18 and Brandon Riker, an entrepreneur and small business owner who worked on President Obama’s 2008 campaign, ended up at 12%.

Other key U.S. House races in California

In the Central Valley, Prop 50 redrew the seat held by Republican Rep. David Valadao to make it even more Democratic. Valadao is a survivor of several targeted Democratic campaigns and one of two remaining Republican House members who voted to impeach Trump after the Jan. 6, 2021, attack on the U.S. Capitol.

He’s expected to make it to the general election, so the primary will determine which Democrat faces him — state Assemblywoman Jasmeet Bains, a moderate backed by the Democratic Congressional Campaign Committee, or Randy Villegas, a political science professor at College of the Sequoias and a school board member who represents the party’s liberal wing.

The schism between establishment Democrats and a younger, insurgent progressive wing is a defining characteristic of many of this year’s primaries.

In a safe Democratic district in San Francisco, Scott Wiener, a state lawmaker and former member of the San Francisco Board of Supervisors, is considered likely to make the November race to replace retiring former House Speaker Nancy Pelosi. The suspense is over whether he will face Saikat Chakrabarti, a wealthy former technology entrepreneur who supported Rep. Alexandria Ocasio-Cortez’s insurgent primary in 2018, or Supervisor Connie Chan, who was endorsed by Pelosi.

In Sacramento, city council member Mai Vang is challenging 81-year-old Rep. Doris Matsui, who succeeded her late husband after he died in 2005.

Rep. Brad Sherman, whose Southern California district stretches from the San Fernando Valley to Malibu, is being challenged by Democrat Jake Levine, a 42-year-old lawyer who argues that it is time to move on from the 15-term congressman.

And in a redrawn district that stretches from Napa Valley into conservative Northern California farming communities, 14-term Democratic Rep. Mike Thompson has drawn a younger challenger, former venture capitalist Eric Jones.

In the Sacramento suburbs, Rep. Kevin Kiley, who left the GOP to become an independent and a critic of partisan gerrymandering, hopes to survive in one of the two Democratic-leaning districts where his more conservative district’s voters were scattered.

Meanwhile, in the San Francisco suburbs, six Democrats and two Republicans are running for the seat formerly held by Democratic Rep. Eric Swalwell, who resigned and ended his gubernatorial bid amid sexual harassment allegations. The top two vote-getters advance to the November ballot to fill the seat starting in 2027, while a special election will be held June 18 for the remainder of Swalwell’s current term.

The Associated Press’ Nicholas Riccardi contributed to this report.