A Democratic super PAC is making a surprise major investment in the Wisconsin Senate race in the election’s final days, a sign that a race many Democrats had long considered a sure victory has become unexpectedly tight. 

Senate Majority PAC plans to spend $2 million on behalf of Democratic former Sen. Russ Feingold, who faces incumbent Republican Sen. Ron Johnson. The ads will run on broadcast and cable TV. 

Since the election cycle began, even many Republicans had believed that Johnson was likely to lose his re-election battle. The freshman senator, who unseated Feingold six years ago, trailed the Democrat in nearly every poll of the race this year, sometimes by as many as 10 points or more.

Democrats stressed that the ad buy was a precautionary step, made possible in part by the group’s record fundraising. (It raised more than $19 million in the first 19 days of October alone.) Party strategists say they believe Feingold is still on track to win. 

[Roll Call’s 2016 Election Guide: Senate]

But the GOP lawmaker has run a campaign designed to make him look like the outsider against Feingold, who served three terms in the Senate before his loss in 2010. In a year when public dissatisfaction with the status quo is near all-time highs, that approach has proved unusually effective for candidates from either party. 

“Republican special interests are on track to spend more than $10 million attacking Russ Feingold because they want to keep Ron Johnson in the Senate to do their bidding at Wisconsin’s expense,” said Shripal Shah, a spokesman for the Senate Majority PAC. “Feingold remains positioned to win, but we aren’t going to take any chances by letting these attacks remain unanswered.”

Chris Martin, a spokesman for the pro-Johnson super PAC Reform America Fund said: “Similar to 2010, it seems that hubris will be the ultimate downfall of Sen. Feingold once again. National Democrats all but called this race over three weeks ago — and now they’re frantically rushing back in an attempt to save Feingold as Wisconsin voters are reminded why they already rejected his radical liberal agenda.”

The Rothenberg & Gonzales Political Report/Roll Call rates the race as Leans Democratic. 

[The Most Effective Attack of the 2016 Election Cycle]

The activity in Wisconsin mirrors a larger trend across the map in congressional races this year: Though Democrats are expected to do well on Election Day, they have struggled to close out races they were long expected to win.

In Iowa’s 1st Congressional District, for example, a race Democrats have long considered one of their top pickup opportunities, the the super PAC Priorities USA was forced to make a large, last-minute investment to help push the Democratic nominee over the top. 

Democrats point out that even with the ad buy in Wisconsin, they’ve been able to expand the map of winnable Senate seats in places like Indiana and Missouri. On Thursday, the Senate Majority PAC also slated more spending for Florida, a state it had previously abandoned.