In 2016, the successful members of the GOP’s class of 2010 U.S. Senate recruits will again face their state’s voters when they seek reelection to the upper chamber of Congress. There is some debate as to which Republican Senator might be the most vulnerable in the next election cycle, one that will draw a general electorate that is more favorable to Democrats than a midterm cycle does.

Of the most vulnerable three Republican Senators of the class of 2010 – Sens. Mark Kirk (R-IL), Pat Toomey (R-PA) and Ron Johnson (R-WI) — some savvy political analysts are convinced that Johnson is the most vulnerable. He is by far the most doctrinally conservative of these three Republicans, and his is perhaps the most out of step with his state’s presidential electorate. The Badger State, though trending ever so slightly red, has remained reliably dark blue in general election years. Johnson’s colleague, Sen. Tammy Baldwin (D-WI), has been dubbed by National Journal the most liberal U.S. Senator in the Congress on social matters.

So, it’s fair to say that Johnson is perhaps the most vulnerable Senator up for reelection in 2016, and that condition was only made worse by the announcement on Thursday that former Sen. Russ Feingold will run again for his old seat in a 2010 rematch.

“People tell me all the time that our politics and Washington are broken. And that multi-millionaires, billionaires and big corporations are calling the shots,” Feingold says in the video. “They especially say this about the U.S. Senate, and it’s hard not to agree. But what are we going to do? Get rid of the Senate?

“Actually, no one I’ve listened to says we should throw in the towel and give up — and I don’t think that either,” he adds. “Instead, let’s fight together for change. That means helping to bring back to the U.S. Senate strong independence, bipartisanship and honesty.”

Feingold is a progressive Democrat in the mold of Sen. Elizabeth Warren, he will generate significant campaign contributions from the nation’s liberal donor base, and he will benefit from an energized liberal electorate in Wisconsin.

Perhaps the only outstanding question that relates to Feingold’s viability is the level of financial support the national Republican committees will commit to aiding Republican candidates in Wisconsin. The Republican National Senatorial Committee has its hands full defending a number of vulnerable members, and the demands of triage might force the committee to chalk Johnson’s race up to a loss early. Conversely, the Republican National Committee and the ultimate GOP nominee will be looking closely at Wisconsin as a state worth trying to flip, and the GOP nominee might have coattails in Wisconsin. Those are both big hypotheticals at this early stage of the race.

Moreover, the same might be said of Pennsylvania where Republicans arguably have a better chance of making Electoral College gains. It’s not clear Johnson’s performance in Wisconsin will track closely with the ultimate GOP nominee whereas that is more likely to be the case in the Keystone State.

Johnson was always fated to have a rough election cycle, but he’s drawn a serious challenger in the former incumbent Democrat. If he wasn’t the most vulnerable Republican up in 2016 before, he probably is now.