U.S. Sen. Ron Johnson (left), a Republican, faces an election challenge from former U.S. Sen. Russ Feingold (right). Credit: Associated Press

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For Republican U.S. Sen. Ron Johnson, it's now all about the ground game.

Johnson announced Monday the launch of a "Badger Brigade" of supporters who have committed to his campaign against Democrat Russ Feingold.

Johnson's campaign claimed the recruiting effort has generated more than 100,000 grass-roots endorsements. Since early 2015, Wisconsin Republicans have made more than 750,000 volunteer voter contacts through phone calls and door knocks, officials said.

During the summer, Democrats and Republicans are expected to gear up their get-out-the-vote efforts in preparation for the fall election.

The Johnson-Feingold race, a rematch of their 2010 contest, is seen as a pivotal contest for control of the U.S. Senate. Polls show Feingold leading.

Even though Johnson has received ample support from third-party groups, the Johnson-Feingold race wasn't part of a $38.6 million ad buy announced Tuesday by the Senate Leadership Fund, the principal GOP Senate super PAC.

A Marquette University Law School Poll released earlier this month had Feingold leading Johnson among registered voters, 45% to 41%. Among likely voters, Feingold's support expanded to 51% over Johnson's 42%.

In a survey released Tuesday by Public Policy Polling, a Democratic firm, Feingold, a former three-term U.S. senator, was backed by 50% of respondents and Johnson by 37%. The survey of 843 registered Wisconsin voters was conducted Wednesday and Thursday and commissioned by the liberal group Americans United for Change.

Thirty-seven percent of those surveyed said they were Democrat, 26% said they were Republican and 37% said they were independent.

Ward Baker, executive director of the National Republican Senatorial Committee, is banking on the GOP ground and data operations to boost Johnson. He pointed to Gov. Scott Walker's three victories and large Republican majorities in the state Senate and Assembly as signs that "Wisconsin is getting redder."

In a teleconference with reporters, Baker brushed off polls that show Johnson behind.

"Ron Johnson has actually been working and keeping his head down and getting things done," he said. "When you talk about the polls, everything we see is margin of error."

Baker sought to portray Feingold as a career politician, adding that the only person more liberal than Feingold on national defense was independent U.S. Sen. Bernie Sanders of Vermont.

Baker said of Feingold: "He's a man without a core and that's why he's running a campaign without a real message."

Harry Hartfield, a spokesman for the Democratic Party of Wisconsin, criticized Johnson's record on taxes, trade and the minimum wage and said that on foreign policy, "Senator Johnson is as reckless as Donald Trump," the presumptive GOP presidential nominee.

"Both The Ronald and The Donald want to send tens of thousands of American troops into another ground invasion in the Middle East. It's no wonder the GOP is starting to panic," Hartfield said in a statement.