Claiming the Democratic nomination to become the next U.S. senator from Wisconsin — and the first openly gay U.S. senator — just got easier for Rep. Tammy Baldwin (D-Wisc.) now that a potential major opponent has announced he won’t seek office in 2012.

Former U.S. Sen. Russ Feingold, who lost his seat during the Republican wave in the 2010 election, said in a message to supporters last week that he wouldn’t run for office in 2012.

“I am grateful for the friendship and support of so many fellow Wisconsinites who suggested I consider running for statewide office in the coming months,” Feingold wrote. “While I may seek elective office again someday, I have decided not to run for public office during 2012.”

Feingold, who since his departure from the Senate founded the group Progressive United, said he instead wants to devote his time to teaching at Marquette University Law School and working to overturn Citizens United, a 2010 U.S. Supreme Court decision allowing unlimited corporate funding for independent political broadcasts in political campaigns.

He was seen as the favorite to win the Democratic nomination — and likely the seat itself — for the seat Sen. Herb Kohl (D-Wisc.) will vacate upon his retirement at the end of next year. With Feingold out of the picture, political observers say Baldwin, who has said she’s “very likely” to pursue a run for Senate, is the front-runner to claim the Democratic nomination. The only out lesbian in Congress, Baldwin has been serving in the U.S. House since 1999.

In a statement to supporters, Baldwin praised Feingold for being what she called “one of the true legends of Wisconsin’s progressive tradition” and said she expects his “political courage” to continue to impact Wisconsin and the country for years to come.

“Lots of you have asked me whether Russ’ announcement will influence my plans,” Baldwin said. “As I’ve said, I’m seriously exploring a race for the U.S. Senate — and I’ll have more to say about that soon.  But whoever represents our party in that important election should have the same progressive principles — and the same courage to do what’s right — that Russ Feingold has displayed every day of his distinguished career.”

Baldwin was expected to hold off on announcing any decision to run for U.S. Senate until after the Wisconsin special elections, which took place Aug. 9, and after Feingold revealed his intentions for 2012. Now that the election is over and Feingold has announced his decision, Baldwin is widely expected to make an announcement just after Labor Day.

Denis Dison, spokesperson for the Gay & Lesbian Victory Fund, which has been pushing Baldwin to run for the Senate, said Feingold’s announcement is “encouraging.”

“We’re still assuming that there is going to be a competitive primary; somebody is going to pop up,” Dison said. “But I think if her decision had much to do with whether or not Feingold was running, obviously this is a much more encouraging environment and atmosphere to run in.”

Larry Sabato, a political scientist at the University of Virginia, said via e-mail that Feingold’s decision to sit out the race is a “big boost” for Baldwin.

“She could not have gotten the nomination against Feingold — no question he would have defeated her if she had even run, which I doubt,” Sabato said. “Now, she’s got a good chance to be the Democratic nominee, although we have to wait and see who runs against her. The dust hasn’t settled from Feingold’s announcement.”

Other Democrats who are said to be mulling potential bids for the Senate seat include Rep. Ron Kind (D-Wisc.), a seven-term member of the U.S. House, and Steve Kagen, a former U.S. House member from Wisconsin who was unseated in 2010. Kind has publicly said he’s considering jumping in the race.

But according to data published last week from Public Policy Polling, Baldwin would defeat those opponents in a Democratic primary. In a three-way race with Kind and Kagen she leads with 37 percent to 21 percent for Kind and 15 percent for Kagen. Additionally, in just a two-way race with Kagen she leads 48-19.

In addition to favorable polling numbers in the hypothetical primary, Baldwin also has more money on hand compared to either Kind or Kagen. In the most recent Federal Election Commission reports, Baldwin posted $1.1 million in cash on hand after raising more than $600,000 thus far this election cycle. Comparatively, Kind has $478,000 in cash on hand after raising $592,00o this cycle. Kagen has no cash on hand and has only raised $18,000 this cycle.

Dison said potential Democratic challengers to Baldwin will look at those numbers in determining whether to run against her.

“I think if anybody who’s going to consider getting into the race will look at that polling, they’ll look at her fundraising and decide whether it will too much of an uphill battle to challenge her,” Dison said.

But winning the seat against a Republican contender in the general election will be more challenging. Potential GOP opponents — like former Gov. Tommy Thompson or former U.S. Rep. Mark Neumann — are marginally ahead of her in the polls.

The data from PPP shows that in a match-up between Neumann and Baldwin, Neumann would win 44-40, although 15 percent of responders said they were undecided. In a contest between Thompson and Baldwin, Thompson would win 50-42, although eight percent of voters identified as undecided. Fundraising data for Thompson and Neumann wasn’t available on the FEC website.

Sabato said the key for the general election is whether Thompson wins the GOP nomination and, if he does, how handily he wins the Republican mantle.

“He’s viewed as a moderate within the GOP, and as we saw in 2010, that can cause problems,” Sabato said. “Will the Tea Party back Mark Neumann or some other opponent of Thompson? Will Gov. Scott Walker and Sen. Ron Johnson decide to endorse Thompson or an opponent in the GOP primary?”

If Thompson clinches the Republican nomination without too much difficulty, Sabato said he’d give him a slight edge over Baldwin in the general election, but added his prediction could be off because of the timing of the Senate race.

“I hasten to add that Wisconsin is going to be a real battleground presidentially,” Sabato said. “Obama’s large majority in 2008 is less representative of Wisconsin’s contested nature than the 2000 and 2004 presidential results, which were extremely close. Presidential coattails could matter greatly in Wisconsin, as in some other Senate contests. And look at the recent Wisconsin State Senate recall elections — $30 million plus spent on a handful of local races, with emotions running very high.”

Sabato said “things are so unclear on both sides” in the Wisconsin Senate race that his Crystal Ball website will rank the contest as a “Toss Up” in its next edition.