The RNC plans to spend at least $4 million on field deployment across the five states by Election Day, according to an aide briefed on the decision. It's an unusual investment, since the committee typically focuses its field resources exclusively on swing states in presidential election years.

Republican officials say they've been planning the blue state effort for months — long before Sanders emerged as a Democratic frontrunner — and insist the Democratic primary had no bearing on their decision. The RNC's massive, seven-to-one financial advantage over the Democratic National Committee, aides say, has afforded them the luxury of investing in states that aren’t in play in the presidential contest.

Still, the move comes as senior Democrats worry that Sanders could jeopardize their House majority. Particularly concerning to Democrats is the prospect that the self-described democratic socialist could undercut them in the affluent suburban areas that powered them in 2018.

Sen. Bernie Sanders. | Steve Helber/AP Photo

During an appearance on “Fox News Sunday“ following his South Carolina Democratic primary win, Joe Biden said the Vermont senator would make "it very difficult down-ballot" for the party.

"This is no great secret to anybody: You can't run as an independent socialist, now a democratic socialist, and expect to do very well in the states we have to win," Biden said.

Sanders aides have pushed back against the idea that he would hurt other Democrats on the ballot, arguing that he would draw a wave of young voters to the polls.

Republicans, however, have already begun to attach Democrats to the Vermont senator — a strategy aimed at suburbanites who've fled the GOP during Trump’s presidency but who may be repelled by Sanders’ politics. The National Republican Congressional Committee, for instance, sent out a press release recently asking whether a pair of vulnerable New Jersey Democrats agreed with comments Sanders made praising Cuban dictator Fidel Castro.

Democrats insist they'll have more than enough money to defend incumbents seeking reelection, noting that they launched a nationwide field program nearly a year ago. As part of the effort, Democratic field staffers have been dispatched to the four blue states Republicans are now targeting.

Democrats also note that their endangered House members have vastly outraised their Republican rivals and have substantial resources to defend themselves.

“While Republicans are just showing up now, DCCC organizers have been on the ground since March of last year laying the groundwork to not only protect, but expand this House majority that is standing up for everyday folks and making sure they have a voice in Congress,” said Robyn Patterson, a Democratic Congressional Campaign Committee spokeswoman.

The four blue states are expected to take center stage in the 2020 fight for the speaker’s gavel. In 2018, Democrats netted seven seats in California, two in Illinois, four in New Jersey and three in New York. Collectively, they represent more than a third of the 41 seats Democrats gained.

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The Republican deployment is part of a broader expansion of the RNC’s field program. By early March, the committee will have more than 800 field staffers on the ground across 23 states. The lion’s share of the party’s efforts are focused on presidential battlegrounds like Michigan, Wisconsin, and Pennsylvania.

In addition to its investment on field deployment, the RNC plans to spend millions more on contacting voters in those states.

While Trump is focused primarily on his own reelection, he's also turned his attention to House races in the wake of the Democratic-led impeachment push. During a post-impeachment speech last month, the president said he was “going to work hard” on congressional contests and that he intends to “get out to those Trump areas that we won by a lot — and you know, in ‘18, we didn’t win.”

“In the aftermath of House Democrats’ impeachment sham,” said RNC chief of staff Richard Walters, “our mission to take back the House is more critical than ever."