Republicans are hoping for a sweep Tuesday in special congressional elections being held in California and Wisconsin.

Meanwhile, Nebraska becomes the first state to hold in-person voting during the coronavirus pandemic since Wisconsin grabbed national headlines by holding a controversial primary early last month.

The contest grabbing the most attention is the special election in California’s 25th Congressional District.

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President Trump and the National Republican Congressional Committee are claiming Democrats are trying to steal the election with a last-minute move to open up an additional polling station in the district – which is located in the northern suburbs of Los Angeles and in the neighboring high desert Antelope Valley cities of Lancaster and Palmdale.

The district was long held by Republicans until Democrat Katie Hill’s victory in the 2018 midterm elections, as the area’s electorate became younger and more diverse. But less than a year into her term, Hill resigned amid controversy involving intimate photos of her that were published online without her consent, and after a House ethics probe looked into accusations of an improper relationship between the congresswoman and a staff member.

The runoff election to replace her, following a first-round nonpartisan vote in early March, is between Republican Mike Garcia – a Raytheon executive and former Navy fighter pilot – and Democratic state Assemblywoman Christy Smith.

Amid the coronavirus outbreak, Los Angeles and Ventura county officials mailed all registered voters in the district an absentee ballot so that they wouldn’t have to vote in person. But there are still polling stations open on Election Day.

“Dems are trying to steal the Mike Garcia Congressional Race in California,” Trump tweeted Monday morning, “Republicans, get out and VOTE for your terrific candidate, ASAP!”

Trump’s plea came after Fox News first reported over the weekend that NRCC Chairman Rep. Tom Emmer of Minnesota called on his GOP colleagues to "raise hell" about a new voting center in a more Democratic area of the district.

"We're issuing an urgent call to arms regarding the vote-by-mail CA-25 special election happening this Tuesday," Emmer wrote to his fellow Republicans.

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Garcia and top Republicans have been crying foul about the county's decision to open up the new in-person voting location. They argue Democrats pushed for the extra voting location after seeing mail-in ballot returns were favoring the GOP.

The Democratic Congressional Campaign Committee (DCCC) rejected Trump's objection, tweeting that "President Trump and Washington Republicans know that the more Californians who have access to the ballot, the worse their chances are -- that’s why they are falsely crying foul on the weekend before the #CA25 special election.”

Democrats have accused Trump and the GOP of trying to restrict voting access. They argue the additional early-voting center was needed so voters in the diverse city of Lancaster wouldn't be disenfranchised. The city’s Republican mayor – while backing Garcia – asked to open the additional voting center.

Tuesday's election will fill out the remainder of Hill's term, which ends next January. Both candidates are expected to face off against each other in the November general election for a full two-year term to represent the district.

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The coronavirus pandemic is the central issue in the race, with Garcia supporting Trump’s repeated calls to reopen the economy as quickly as possible and Smith backing California Democratic Gov. Gavin Newsom’s plan for a more measured opening of the economy.

Democrats are concerned about losing the seat – which they just flipped two years earlier – during what’s expected to be a low turnout special election contest amid the coronavirus pandemic. But they’re confident that if they lose Tuesday’s vote, they’ll regain the district during what should be a much larger general election turnout in November.

For the GOP, a victory in Tuesday’s special contest – in a district won by Hillary Clinton in 2016 – will give them bragging rights that they can win in a suburban swing district and encouragement and energy for their base heading into November as they face the tough task of regaining the majority in the House of Representatives.

Wisconsin showdown

There’s a lot less drama in the other special election – in Wisconsin’s 7th Congressional District – which is solidly Republican.

The race is to fill the seat left open by 5-term GOP Rep. Sean Duffy – a major Trump supporter who stepped down from Congress last September to spend more time caring for his newborn baby, who was born with health complications. Duffy – who became famous starring on the MTV reality series “The Real World” in 1997 and was a professional lumberjack athlete as well as a county district attorney – won the district by 22 percentage points in his 2018 reelection.

And Tuesday’s election between Republican state Sen. Tom Tiffany and Democratic school board official Tricia Zunker is all about the margin of victory. Tiffany is expected to easily win, but Democrats will be encouraged if they can keep the margin in the single digits.

The contest for the district, which covers parts of 26 counties in the northern part of Wisconsin, is being held both by mail-in ballots and in-person voting – with state National Guard troops helping out at some polling locations.

Nebraska primary

It’s primary day in Nebraska, where the state becomes just the second in the nation to hold a predominantly in-person contest during the coronavirus pandemic – following Wisconsin.

Following a bitter partisan battle, Wisconsin went ahead with in-person voting – and pictures of people waiting in long lines and unable to keep a safe distance from each other grabbed national attention. Officials say that more than 50 voters or polling workers contracted the coronavirus during the primary.

There’s much less drama in Nebraska.

Republicans – who hold all statewide offices and control the legislature – have been encouraging people to vote early through absentee balloting. And officials say an unprecedented 300,000 people have already requested absentee ballots. But they note that state law requires that polling stations be open.

On the ballot is the Democratic presidential nomination, as well as a battle between the Democratic Party’s centrist and left wings in a primary for a competitive House seat controlled by the GOP. There are also down-ballot races.

Fox News’ Marisa Schultz and Ronn Blitzer contributed to this report.