Senate Republicans are eyeing a last-minute comeback in a state long written off as a loss: Wisconsin.

The sleeper race is turning into an unexpected 11th hour battleground with Sen. Ron Johnson Ronald (Ron) Harold Johnson CIA found Putin 'probably directing' campaign against Biden: report This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report MORE within striking distance of his Democrat opponent, former Sen. Russ Feingold.

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Feingold is still ahead in polling, but his lead has shrunk from the double-digit average he held for most of the summer. The former senator is clinging to a 1-point lead in a Marquette Law School poll released Wednesday but trailing Johnson among independent voters.

"This race is a dead heat and the momentum is clearly with Ron Johnson. Wisconsinites are closer than ever to sending Senator Feingold back to California — for good,” said Brian Reisinger, a spokesman for Johnson’s campaign.

Republicans argue the race is even closer than what public polling — which has Feingold averaging a nearly 5-point lead — suggests. A GOP source said their polling shows the race tied.

Meanwhile, a source familiar with the Johnson campaign’s strategy added separately that their internal modeling shows the GOP senator gained a slight lead over the fall, with numbers continuing to move in Johnson’s favor heading into the final week.

The polling comes as the nonpartisan Cook Political Report moved the race from “lean Democrat” to “toss up.”

An upset in Wisconsin would likely reverberate across the battle for the Senate, spelling trouble for Democrats’ efforts to regain the upper chamber.

Democrats long bet on the Badger State being in the win column as they plot their path to picking up the five seats — or four if they retain the White House — they need to win back the majority.

Sensing the potential for an upset, outside groups are pouring in money as the race nears the finish line.

The Senate Leadership Fund, a GOP outside group with ties to Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.), dropped $2 million into the Senate battle, arguing polling shows that Republicans could squeak out a win.

“We think there is a solid opportunity to take a race that the Democrats took for granted and thought they had locked up in July,” said Ian Prior, a spokesman for the group.

The GOP-aligned Americans for Prosperity, the Let America Work PAC, America’s PAC, the U.S. Chamber of Commerce and the NRA Political Victory Fund are all dropping money in the final week of the election, according to a Republican watching the advertising market.

In total, the six GOP groups are on track to spend nearly $4.2 million in the home stretch of the Wisconsin race.

The help comes after Johnson was largely abandoned by outside groups for months even though his campaign consistently argued the race was closer than national groups gave them credit for.

The senator — a former businessman elected in a wave year for Republicans — has worked overtime to paint Feingold as a Washington insider in a year that has favored outsiders. Feingold previously spent 18 years in the Senate.

His campaign also worked to humanize the straight-laced businessman, using TV ads to tout his work with a Milwaukee-based anti-poverty program and his effort to help a family bring their adopted daughter home from the Democratic Republic of the Congo.

A source familiar with Johnson’s strategy noted the two ads helped “bring home” Republicans who may have sat out on the 2016 election because of frustration with the presidential race.

His campaign is poised to spend more than $1.5 million in the final week of the race on TV and radio ads, according to a Republican watching the advertising market.

Johnson's GOP colleagues have also poured into the state as race has tightened. Sens. Joni Ernst (Iowa), Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (S.C.) and David Perdue (Ga.) stumped for the Wisconsin senator over the past week.

Johnson campaigned with Republican presidential nominee Donald Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE for the first time on Tuesday night and is expected to kick off a bus tour Thursday with Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (Wis.).

But Democrats remain confident they’ll emerge victorious on Tuesday night.

Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee, said Republicans are spending millions attacking Feingold “because they know they can't run on Senator Johnson's record — especially not when he's stumping with Donald Trump.”

They counter talk of GOP momentum by noting that every public poll in the race save one shows the same result: Feingold winning the race.

Democrats concede that the Senate race has tightened but say the higher voter turnout typically seen in presidential election years gives them an inherent advantage.

“We expect this to be a close race straight through Election Day, and we will not take our foot off the gas,” Tom Russell, Feingold’s campaign manager, said in a note to supporters. “We’ve got a great [get-out-the-vote] effort underway, and with a push through Election Day, we’ll all get to celebrate a big win.”

Nearly 568,000 Wisconsinites have already voted, according to the state’s election commission, putting 2016 on pace for record turnout in the state.

NBC News estimates that 52 percent of early voters are Democrats, compared to 36 percent who are Republicans. The Marquette poll released Wednesday also had Feingold winning early voters 58 percent to 29 percent.

Russell added “whether you look at the analytics or the geographic breakdown, Democrats are voting and Republicans are suffering an enthusiasm gap for both Senator Johnson and Donald Trump.”

Even if they believe the state’s dynamics undercut Johnson’s chances, Democrats aren’t taking any chances in the home stretch.

Feingold stumped with Sen. Tim Kaine Timothy (Tim) Michael KaineButtigieg stands in as Pence for Harris's debate practice Trump meets with potential Supreme Court pick Amy Coney Barrett at White House Names to watch as Trump picks Ginsburg replacement on Supreme Court MORE (D-Va.), Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE’s running mate, and Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) this week. Sen. Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D-Ore.) — Sanders’s lone Senate endorser his Democratic presidential primary bid — will be in the state Thursday.

Feingold’s campaign is also expected to outspend Johnson in the final week. It is poised to drop more than $2.3 million on TV and radio ads, according to a Republican watching the advertising market.

The Senate Majority PAC, which has ties to Senate Minority Leader Harry Reid Harry Mason ReidThe Supreme Court vacancy — yet another congressional food fight Trump seeks to turn around campaign with Supreme Court fight On The Trail: Battle over Ginsburg replacement threatens to break Senate MORE (D-Nev.), is also injecting $2 million in the final week of the race. Feingold has faced nearly $9.5 million in negative spending from outside groups, according to the Center for Responsive Politics.

Shripal Shah, a spokesman for the group, said “Feingold remains positioned to win, but we aren't going to take any chances by letting these attacks remain unanswered."

Reid Wilson contributed.