EXCLUSIVE: The leader of the National Republican Congressional Committee (NRCC) sent a memo to all House Republicans Saturday with an "urgent call to arms" that Democrats are trying to "steal" Tuesday's special election for California's 25th Congressional District seat, Fox News has learned.

President Trump echoed the concerns of NRCC Chairman Tom Emmer, R-Minn., on Twitter Saturday and blasted the "rigged election" in the northern Los Angeles County district.

"Governor [Gavin Newsom] of California won't let restaurants, beaches and stores open, but he installs a voting booth system in a highly Democrat area (supposed to be mail in ballots only) because our great candidate [Mike Garcia] is winning by a lot. CA25 Rigged Election!" Trump wrote.

Tuesday's special election is between Republican Mike Garcia and Democrat Christy Smith. The winner will fill out the term of freshman Rep. Katie Hill, D-Calif., who resigned in October 2019 amid a "throuple" ethics scandal.

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Garcia and top Republicans are crying foul about the county's decision to open up a new in-person voting center Saturday in a more Democratic area of the district. They argue Democrats pushed for the extra voting location after seeing mail-in ballot returns were favoring the GOP.

Democrats, however, have blasted Trump and the GOP for 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.

There's currently a total of 10 voting centers open for the special election.

Fox News exclusively obtained a memo sent by Emmer to House Republicans on Saturday urging them to "raise hell" about the new voting center.

"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.

NRCC Chairman Tom Emmer (R-Minn.)

The GOP memo says that as of Friday, 50,191 Republicans (45 percent) have returned their ballots compared to 39,230 Democrats (35 percent) and 16,528 (15 percent) from unaffiliated voters. Republicans argue the Democrats were trying to make up ground by bringing a last-minute voting center to a more left-leaning community.

Los Angeles County election officials announced the Lancaster voting center on Friday that will be open Saturday, Sunday and Monday for early voting. But Emmer said the Democrats in Lancaster already had access to a voting center in Palmdale, "which is a 10-minute drive from the new Lancaster center."

"It is very clear the Democrats are trying to steal this election and we need your help raising hell on social media, TV, radio, etc." Emmer wrote in the memo Saturday.

Mike Garcia (R) and Christy Smith (D) are competing Tuesday to represent California's 25th Congressional District.

With the coronavirus pandemic health concerns, Gov. Gavin Newsom issued an executive order March 20 requiring all registered voters to receive ballots in the mail to allow them to stay home and still participate in Tuesday's election. In-person voting options were still required and would allow same-day voter registration opportunities, too.

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But Democrats in Lancaster were raising concerns they didn't have a voting center in their diverse community and that would cause poor turnout for African-Americans, according to LAist.com. The mayor of Lancaster, R. Rex Parris, a Republican and supporter of Garcia, also wanted an additional vote center.

"This is something the county should fix immediately," Parris told LAist.com. "There should not be even the appearance of affecting the outcome by limiting the ability to vote."

"I want Mike to win, I think he'd be a better congressman, but I don't want to 'jimmy' the election," Parris added.

But in the memo, Emmer pushed back on any suggestion that Republicans are trying to disenfranchise minority voters.

Emmer suggested the following response to GOP members: "Every single registered voter was mailed a ballot. Is the implication that minority voters do not have mailboxes?"

Turnout is going to be especially important in this swing district.

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The race is already in the national spotlight because Hill flipped the seat to Democrat control in the 2018 election amid the "blue wave." Now, Republicans are determined to win it back in a tight race.

"We are on the verge of shocking the world by flipping a California congressional seat for the first time since 1998 that a Democrat won by 9 points in 2018," Emmer wrote, referencing Hill's victory two years ago. "Democrats know this and they are doing all they can do to steal the election."

Trump has inserted himself in the mix by giving his full backing to Garcia, a former Navy combat pilot. While Smith, a Democratic state legislator, netted the endorsement of former President Barack Obama and former Secretary of State Hillary Clinton.

The winner Tuesday will fill out Hill's term through January 2021. There will be another election in November for the full two-year term starting in 2021.