Feingold spent 18 years representing Wisconsin in the Senate before losing to Johnson, a tea party-affiliated businessman, by 5 percentage points in 2010. But the Democrat starts the 2016 race with an apparent advantage over Johnson: A Marquette Law School poll last month found Feingold earning 54 percent of the vote, to 38 percent for the incumbent.

“Let’s fight together for change,” Feingold says in an announcement video filmed at his home in Middleton, Wisconsin. “That means helping to bring back to the U.S. Senate strong independence, bipartisanship and honesty. So, today, I’m pleased to announce I’m running for the U.S. Senate in 2016.”

The Democrat’s entrance into the race is not unexpected. Feingold left a State Department position as a special envoy to the Great Lakes region of Africa in February.

His decision to run is a victory for national Democrats who hope to take back the Senate in 2016. Rep. Gwen Moore and other Democrats had also considered running, but Feingold’s entrance is likely to clear the field.

Republicans — including Johnson — immediately slammed Feingold as a “career politician.”

“I didn’t start running [for] the Senate until six months out,” he said. “The fact that he’s running so early just proves that he must be so addicted to being a career politician that he just couldn’t wait to get back in here.”

“I mean it’s going to be a tough race, no doubt about it,” Johnson added in an interview with POLITICO. “I’m not scared. Do I look scared?”

Johnson, who has done little to tone down his conservative tendencies in a purple state, didn’t hold back on criticizing his predecessor.

“Russ Feingold spent the previous 30 years trying to grow government, fully committed to a government that’s more intrusive, wants to spend more money, wants to rule your life,” he said. “So it’s going to be a pretty stark contrast.”

Feingold is able to unite the party’s recently feuding establishment and progressive wings as a candidate who is both electable and has a sterling liberal record. The Democratic Senatorial Campaign Committee endorsed Feingold almost immediately, with DSCC chairman Jon Tester calling Feingold “a tenacious champion for the people of Wisconsin throughout his career.”

The Wisconsinite also quickly earned plaudits from Democracy for America, the Progressive Change Campaign Committee and Sen. Elizabeth Warren (D-Mass).

“I’m happy, excited, thrilled, & all-around ecstatic that @russfeingold is running to return to the Senate,” she wrote on Twitter.

Feingold enters the race in a remarkably strong position for a challenger. Not only does the Marquette poll show him with a 16-point lead over Johnson, it also shows he has higher name recognition and favorability ratings. Forty-seven percent of registered voters have a favorable opinion of Feingold, compared with 26 percent who view him unfavorably. The split for Johnson was 32 percent to 29 percent.

In the announcement, Feingold never mentions Johnson by name. He does mention a long-time policy passion of his: limiting the role of money in politics.

“People tell me all the time that our politics in Washington are broken and that multimillionaires, billionaires and big corporations are calling all the shots,” Feingold says.

Feingold and Sen. John McCain (R-Ariz.) wrote the last major update to the nation’s campaign finance law in the early 2000s. That law has since been gutted by court decisions striking down many of its provisions as unconstitutional restrictions on free speech.

Feingold’s campaign would not make the former senator available for interviews.

Johnson will start the race with $1.5 million cash-on-hand, having raised $1.3 million in the first quarter of 2015. The former plastics company CEO spent $8.8 million of his own money on the 2010 race, but he has expressed reluctance to self-fund this campaign.

He can expect some support from conservative allies: The Club for Growth, a conservative economic group, is also “amassing a war chest” to back Johnson, according to a source familiar with the group’s plans.

Burgess Everett and Seung Min Kim contributed to this report.