Add Russ Feingold to the list of Senate Democrats who find themselves in unexpectedly tough races, the latest evidence of the GOP's success in widening the playing field that President Obama's party has to defend.

The Wisconsin Democrat faces a wealthy political newcomer with early backing from Tea Party activists in a state that has many independent voters and is known for doing its own thing.

Likely GOP nominee Ron Johnson is running an outsider's campaign in a year that seems to favor outsiders.

"We have to boot professional politicians out of Washington," he says in his first campaign ad.

Feingold, now in his third term, knows he has a fight on his hands. Never shy about showcasing his independent streak, he reiterated his splits with the White House and fellow Democrats on two key policies last week.

"Regardless of who is in command, the president's current strategy in Afghanistan is counterproductive," Feingold said as the Senate confirmed Gen. David Petraeus to lead that war.

Feingold also renewed his opposition to the regulatory overhaul that Obama and Democrats wrote for Wall Street. "My test for the financial regulatory reform bill is whether it will prevent another crisis," and the measure "fails that test," he said.

In one of his first campaign ads, Feingold recalls his opposition to the unpopular 2008 rescue of banks and other financial institutions, a measure supported by many Democrats, including Obama. "I said 'No' to the bailout," he says.

His occasional breaks from party orthodoxy could be his key to political survival in a campaign season that has already claimed five congressional incumbents and several establishment-backed candidates.

But, as Republicans are quick to remind voters, Feingold also backed Obama's economic stimulus plan and health care overhaul.

Democrats started the election year anticipating that several Senate incumbents would be vulnerable, including Majority Leader Harry Reid in Nevada, Blanche Lincoln in Arkansas and Michael Bennet in Colorado.

But few expected Feingold, Barbara Boxer in California or Patty Murray in Washington to confront serious challenges. All three face competitive contests.

Republican victories in any or all of the three races would indicate a huge night for the GOP, perhaps big enough to take control of Senate. To do it, they must claim 10 seats from Democrats without losing any of their own -- a tall order.

At the least, more competitive races mean Democrats will have to spend money in places they didn't think they needed it.