Thanks to the close Democratic presidential contest, California’s top-two primary system and a surge of new registered voters who may or may not show up to vote, it’s hard to speculate about what may happen in the state’s congressional races Tuesday, or if the results will even be known that night.

What is known is that many voters are confronted with races that involve vulnerable incumbents, competitive open seats and intra-party fights.

Voters in all 53 districts are considering multiple candidates for U.S. House seats, some more seriously contested than others. The Times will be tracking each race, and here are some of the most interesting.

The top two vote-getters advance to the general election in November, regardless of political party. It’s expected to create at least handful of same-party races this year.

A rematch in Silicon Valley

The rematch between eight-term Rep. Michael Honda (D-San Jose) and former Obama administration official Ro Khanna in the 17th District is one of the most closely watched races.

Honda beat Khanna in 2014 by 3.6 percentage points after a close, expensive and often-bitter race. Honda had broad support among the state's Democratic Party leadership, while many Silicon Valley entrepreneurs lined up behind Khanna.

The district includes heavily Asian communities in Santa Clara, Sunnyvale, Cupertino, Milpitas and part of San Jose and stretches north to Fremont and Newark.

But this time, Khanna has poached several of Honda’s endorsements, and President Obama has chosen not to endorse the congressman again, instead staying out of it.

Also new this year is an ethics complaint against Honda, that is still being investigated by the House Ethics Committee.

The House Office of Congressional Ethics determined there is “substantial reason to believe” that Honda and his congressional staff used taxpayer resources to benefit his campaign.

The separate House Ethics Committee also is investigating the allegations.

Nathan Gonzales, editor of the nonpartisan Rothenberg & Gonzales Political Report newsletter, said that how voters turn out for Honda and Khanna on Tuesday will suggest a lot about Honda’s reelection chances.

“Because it’s a rematch, we’ll have a direct comparison … and see the position he’s in going into November,” Gonzales said.

Top legislative races in California: What we're watching >>

Taking a chance on an opening

California has four congressional seats without an incumbent this year, including three that were held by the same person for decades.

Seeing what might be a once-in-a-lifetime opportunity, more than a half-dozen people are seeking each seat.

The 24th District, now represented by Democratic Rep. Lois Capps (D-Santa Barbara), has drawn nine candidates to succeed her.

Among the Republicans are state Assemblyman K.H. "Katcho" Achadjian of San Luis Obispo and Justin Fareed, a 28-year-old former Capitol Hill staffer who works for his family's sports medical devices company and has raised more than $1 million.

Democrats include Santa Barbara County Supervisor Salud Carbajal, whom Capps has endorsed, and Santa Barbara Mayor Helene Schneider, whom Lt. Gov. Gavin Newsom has endorsed.

The district spans nearly 200 miles of coast from Ventura to Ragged Point, north of San Luis Obispo, and runs inland to the Temblor Range at the edge of the San Joaquin Valley and south to Los Padres National Forest. It’s made up of a mix of coastal liberals, college students and conservative inland ranchers and farmers.

In 2014, Republican Chris Mitchum, who is not running this year, came within four points of ousting Capps from the seat she has held since 1998.

Gonzales said so many candidates in the race makes it “primed for a surprise.”

Nanette Barragan and state Sen. Isadore Hall (Barragan for Congress, Gary Friedman / Los Angeles Times)

The race in the 44th Congressional District to replace Rep. Janice Hahn, who is leaving Congress to run for county supervisor, has mostly focused on two Democrats, attorney Nanette Barragán and state Sen. Isadore Hall.

The largely working-class district stretches from the port communities of Los Angeles north through Carson, Compton and Willowbrook to South Gate and Lynwood.

Hall was endorsed early by Hahn, and has the backing of the state Democratic Party and many state, local and federal Democrats.

Barragán is backed by Emily’s List, the League of Conservation Voters and several state, federal and local Democrats.

In the 46th Congressional District, eight candidates are hoping to replace Democratic Rep. Loretta Sanchez, who is running for the U.S. Senate.

The central Orange County district includes Disneyland, the Latino-heavy city of Santa Ana to the south and, to the east, a strip of Little Saigon.

For the first time, some Orange County voters might have to choose between two Democrats for Congress >>

Former state Sen. Lou Correa and former state Sen. Joe Dunn are the top fundraisers to date, though Garden Grove Mayor Bao Nguyen was catching up in the final weeks of the race. All three are Democrats.

Meanwhile, five people are running to fill the 20th District seat along the Central Coast now held by Rep. Sam Farr (D-Carmel). Monterey County Deputy Dist. Atty. Jimmy Panetta, whose father Leon Panetta held the seat before Farr, has dominated in fundraising and is the strong favorite.

Watching San Fernando Valley

Also worth keeping an eye on is the 29th District race, which pits Rep. Tony Cardenas (D-Los Angeles) against three Democrats, former Los Angeles City Council member Richard Alarcon , David Guzman and Joe Shammas and has raised ethical questions.

Cardenas, the only candidate who has raised a substantial amount of money for the race, has poured more than $300,000 of it into legal services with five Los Angeles law firms in the last year, but his attorney says there is no indication the congressman is the subject of a rumored federal investigation.

More than a dozen people who have worked for or contributed to politicians in the San Fernando Valley have been subpoenaed by federal investigators in the last year, including an aide for the congressman's district office who previously worked for him when he was a member of the Los Angeles City Council.

Another twist in the race was the April 22 announcement that Alarcon and his wife, Flora, will be retried on perjury and voter fraud charges.

In 2014, a jury found them guilty of lying about where they lived so that Richard Alarcon could run for a council seat. The state’s 2nd District Court of Appeals threw out the convictions, saying the trial judge had given an improper jury instruction.

Republicans in trouble?

National Democrats have honed in on two Central Valley seats as possible districts to flip, hoping to lump the California Republicans in with businessman Donald Trump, the party’s presumptive presidential nominee who has made bombastic claims about Latinos.

Both Rep. Jeff Denham (R-Turlock) of the 10th Congressional District and Rep. David Valadao (R-Hanford) of the 21st District represent areas with high Latino populations, and have called for changes to the country’s immigration system.

Denham is seeking a fourth term, with his chief opponent being Democrat Michael Eggman, a beekeeper whom he beat in the 2014 general election by 12 points.

Democrat Mike Barkley and Republican Robert Hodges are also on the ballot but haven’t raised money to build substantial campaigns.

The district is 26% Latino, according to the nonpartisan election guide California Target Book, and includes Modesto, Turlock and parts of Stanislaus and San Joaquin counties.

Valadao has built up a war chest of $1.15 million to defend his seat representing the 21st District, which covers parts of Fresno and Kern counties. It is 71% Latino, according to California Target Book.

Rep. David Valadao (Bill Clark/CQ Roll Call)

Who does your member of Congress back for president? >>

The Democrats in the race are Fowler City Councilman Daniel Parra, who had $3,836 in cash on hand, according to campaign filings, and Bakersfield lawyer Emilio Huerta, the son of labor icon Dolores Huerta, who had $54,420 in the bank.

“The Democrats are lucky that Huerta has jumped into the campaign and brings the family name into play,” said University of the Pacific associate political science professor Keith Smith.

Still, while the populations of both Denham and Valadao’s Central Valley districts are shifting, this might not be the year for Democrats in either race.

The nonpartisan political analysis website Cook Political Report lists both districts as leaning Republican.

Democrats also are eager to oust Rep. Steve Knight (R-Palmdale), who beat Republican Ted Strickland with 53.3% of the vote to secure his first term in the 25th Congressional District in North Los Angeles County and Simi Valley in 2014.

This time, Knight faces attorney Bryan Caforio and Los Angeles Police Department Lt. Lou Vince, both Democrats. Republican Jeffrey Moffatt also is running.

The two Democrats have divided their party, with local partisans backing Vince, who has struggled to raise the money needed to take on an incumbent, and national Democrats backing Caforio, who moved into the district last year and wasn’t registered to vote there until November.

Knight's opponents have criticized his response to the natural gas leak in Aliso Canyon that displaced thousands of residents of Porter Ranch and the surrounding community.

The Cook Political Report lists the district as leaning Republican.

As Porter Ranch gas leak lingers, candidates smell a political opportunity >>

Rep. Ami Bera (Chris Maddaloni/CQ Roll Call)

Democrats on defense

Republicans also have targeted seats they see as potential pickups, including Reps. Ami Bera (D-Elk Grove) in the 7th District and Scott Peters (D-San Diego) in the 52nd.

Republicans have repeatedly pressured Bera, a physician and son of immigrants from India, to provide more information about his father Babuel Bera’s guilty plea in May to illegally funneling more than a quarter of a million dollars to his son's 2010 and 2012 campaigns.

The congressman has said that neither he nor campaign aides knew of his father’s activities until they were contacted by federal prosecutors. An aide said Bera wrote a check from his political account to the U.S. Treasury on the day of the plea to cover the entire amount identified by prosecutors.

When seeking a second term in 2014, Bera had five primary challengers. He defeated former Republican Rep. Doug Ose with 50.4% of the vote in what was among the tightest House general election contests in the country.

Bera this year has attracted only one primary opponent: Republican Sacramento County Sheriff Scott Jones.

Bera was already seen as having a tough time with some traditional Democratic supporters after labor unions sharply criticized him for supporting President Obama's Trans-Pacific Partnership trade agreement.

As the only Democrat candidate on the ballot, if Bera gets less than 52% of the vote Tuesday, it could be a signal of trouble ahead for Democrats this fall, said Smith of University of the Pacific.

The Cook Political Report lists the seat as leaning Democratic.

Rep. Scott Peters (Bill Clark / CQ-Roll Call)

Peters represents a district that has been known to oust incumbents.

Peters himself unseated GOP Rep. Brian Bilbray in 2012 to win the district, which includes a chunk of northern San Diego, along with the suburbs of Poway and Coronado, several universities and Navy and Marine Corps bases.

In the 2014 primary, Peters had three Republican challengers and pulled just 42.3% of the vote. In the general election he defeated Republican challenger Carl DeMaio with 51.6% of the vote.

Loyola Law School professor Jessica Levinson said that though the five Republicans on the ballot could splinter any serious opposition, it’s unusual for an incumbent to face so many opponents.

“People smell blood in the water,” Levinson said.

San Diego businesswoman Denise Gitsham, the candidate with the best chance, had $179,103 in cash on hand just weeks before the primary. Peters had just under $1.7 million in the bank.

Gonzales called a Peters loss a long shot, saying he doesn’t think Republicans have much of a chance against the Democrat, who has been boosted by the national party and even Obama the entire campaign cycle.

The Cook Political Report lists the seat as likely to remain in Democratic control.

sarah.wire@latimes.com

Follow @sarahdwire on Twitter

Read more about the 55 members of California's delegation at latimes.com/politics

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UPDATES:

9:02 a.m. June 6: This article was updated with details of a District race in the San Fernando Valley.

This article was originally published at 2 p.m. June 4.