Incumbent Sen. Ron Johnson, R-Wis., faces one of the toughest re-election fights of any Republican in the U.S. Senate for 2016. His race is one of several that will determine whether the GOP keeps its majority.

Johnson's campaign expects the race will be too close to call right up through Election Day, as he faces a rematch against his 2010 opponent, former Sen. Russ Feingold.

Johnson's team wants to prove his 2010 election during a wave of Tea Party-led victories was not a fluke, and for voters in Wisconsin to perceive him as the upstart outsider.

"He might be the incumbent, but he's still the outsider," said Betsy Ankney, Johnson's campaign manager. "He's still running against a three-term career politician."

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While an incumbent running as an outsider might seem odd, he isn't the only Midwestern Republican looking to portray himself as an outsider in a tough reelection fight for a seat in the U.S. Senate. Incumbent Sen. Rob Portman, R-Ohio, who was also first elected in 2010, has gone after his opponent's record of public service and run ads criticizing the Democrat for calling a previous role in Washington his "dream job."

But Johnson has risen quickly in Washington, in Feingold's absence. When Republicans took control of the Senate in 2014, Johnson became chair of the Homeland Security and Governmental Affairs Committee. While he has logged fewer years in the upper chamber than any other committee chairman in the entire U.S. Senate, his experience allows him to tout his leadership in Washington to voters back home.

The most determinative factor in the Wisconsin Senate race, however, may have nothing to do with Johnson's tenure in office. His chances are inextricably tied to the 2016 presidential campaign, and the GOP nominee in particular.

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Wisconsin's governor, Scott Walker, announced his campaign for president on Monday night and the positive attention he receives could boost Johnson and other Republicans' chances at home.

Ford O'Connell, a GOP strategist, said he believes Democrats hope Hillary Clinton's presidential campaign will drag Feingold over the finish line.

"If Clinton is running plus 15 in Wisconsin, Ron Johnson is toast regardless of how great he is," O'Connell said. "Walker will have a tougher time winning in Wisconsin than Johnson would… but their fates would be tied together [if they are both on the ballot]."

But Johnson appears uninterested in riding Walker's coattails. While some of his campaign staffers made the trip to Waukesha, Wis., for Walker's formal announcement on Monday, Johnson headed back to work in Washington on Sunday.

"The people of Wisconsin chose Ron Johnson because he is an accountant and a manufacturer — and not a politician," Ankney said. "That's exactly what the voters wanted after 18 years of a partisan Washington insider like Senator Feingold. Ron is still the accountant and manufacturer from Oshkosh, and Feingold is still the partisan senator from Washington."

While Johnson and Feingold are running neck-and-neck in terms of their fundraising numbers, early polling showed Feingold with a double-digit lead. James Wigderson, a right-leaning columnist for the Waukesha Freeman, thinks Johnson may not survive.

"I think Johnson may get a boost from Scott Walker's candidacy for president, but it's going to be a really tough campaign," he said. "In the end, Feingold was really not that unpopular a figure that Johnson beat."