Former Senator Russ Feingold (D-WI) announced Thursday that he'll make a comeback bid for his old seat. In his announcement video , Feingold said he wants to restore "strong independence, bipartisanship, and honesty" to the Senate. He'll challenge Sen. Ron Johnson (R-WI), who defeated Feingold in 2010 but now trails Feingold by double digits in early polls.

A campaign finance reformer and civil liberties advocate

Feingold didn't get too detailed on the issues in his announcement video, besides stressing that he'd help improve the "economic well-being" of Wisconsinites.

But two big causes close to Feingold's heart — campaign finance reform and civil liberties — seem to be particularly resonant these days.

In the late '90s, Feingold and his Republican colleague John McCain fought to rein in the increasing amounts of money pouring into the electoral system. Eventually, they helped broker a bipartisan deal to pass the "McCain-Feingold" bill in 2002. It closed the loophole allowing unlimited "soft money" donations to national parties, and placed restrictions on ads from outside groups aired in the weeks before federal elections.

But in the years since, the Supreme Court has narrowed the scope of permissible campaign finance regulations further and further, striking down several restrictions on election spending, including important parts of McCain-Feingold. Now the campaign finance reform issue is energizing the left, with even Hillary Clinton suggesting that a constitutional amendment to counteract recent Supreme Court decisions might be necessary.

Second, Feingold has long cared deeply about civil liberties issues, and advocated for reining in NSA surveillance. In 2001, he was the only senator to vote against the Patriot Act. "If we lived in a country that allowed the government to open your mail, eavesdrop on your phone conversations, or intercept your email communications, that country would not be America," he said then. Sitting on the Intelligence Committee, he warned of NSA excesses. Many concluded that his warnings were vindicated after Edward Snowden's revelations.

Feingold looks like the favorite

Feingold's opponent, Sen. Ron Johnson (R-WI) — a businessman new to politics when he took on Feingold in 2010 — looks quite vulnerable. An April poll showed Feingold leading him by 16 percentage points (54 to 38) — very grim numbers for an incumbent. Last fall, the Washington Post named him the most vulnerable incumbent senator up this cycle.

It's no accident that Feingold's announcement stresses bipartisanship —Johnson has built up an extremely conservative voting record during his term. From 2013 to 2014, only six senators were more conservative than Johnson, according to the rankings of political scientists Keith Poole and Howard Rosenthal.

Feingold lost by 5 percentage points in 2010, during the Tea Party wave midterm year. Turnout among the Democratic base is expected to be stronger in the presidential year of 2016. However, if Wisconsin's governor, Scott Walker, wins the Republican nomination, it's unclear what effect that would have on partisan turnout in the state.