Here comes John Fetterman - for Lt. Gov Mike Stack's job | Chris Brennan

Lt. Gov. Mike Stack III appears to be lying low these days.

And that's normally a good thing for the person holding Stack's job. We usually hear about lieutenant governors in Pennsylvania only when something has gone horribly wrong.

But Stack can't hide forever from John Fetterman.

Sources close to Fetterman say he will announce Tuesday that he is challenging Stack's bid for a second term in the Democratic primary election next May.

That will make Fetterman, who just won his fourth term as mayor of Braddock, a small borough just east of Pittsburgh, Stack's third declared challenger.

No disrespect to the others, Chester County Commissioner Kathi Cozzone and Aryanna Berringer of Westmoreland County, but Fetterman has a certain political star power. And that makes him a potent problem for the scandal-plagued Stack.

They couldn't be more different.

Stack, a former state senator and the third generation of a political family based in Northeast Philly, is whip-thin and always nattily attired.

He has gone quiet since news broke in April that he and his wife had berated state troopers and staffers employed to protect and serve them. Gov. Wolf pulled the staffers, Stack apologized and his wife, Tonya, sought in-patient treatment for mental-health issues.

And just like that, Stack went from someone people didn't think too much about to a political liability for Gov. Wolf.

Fetterman, who arrived in Braddock in 2001 for an AmeriCorps job after graduating from the Harvard School of Government, stands 6-foot-8 and covers his tattoo-adorned massive frame in black work shirts and shorts.

He finished third out of four candidates in last year's Democratic primary election for the U.S. Senate but made a name for himself as an authentic populist in a time of so much faux-populism.

And just like that, Fetterman becomes a danger to Stack's political longevity and a tricky balancing act for Wolf.

Fetterman declined to comment Friday about his plans. His personal website was undergoing a tune-up, with just the words "coming soon" on display.

Look for Wolf to stay out of it and let things unfold. He would shed no tears over a defeat for Stack. And Fetterman would add a much-needed boost of energy for a general election campaign.

Stack did not respond to requests for comment. A spokesman for his campaign said Stack has not spoken to Wolf about the candidates challenging him and that he remains focused on his job.

Don't expect Fetterman to spend a lot of time dwelling on Stack's problems. That story tells itself. No need to get in the way of it.

Instead, Fetterman will be talking about Braddock, the 0.65-square-mile burg that rose and fell with the steel industry. Fetterman has drawn national attention for his efforts to reinvigorate the town, where 37 percent of the 2,118 residents live below the federal poverty level.

The New York Times, Rolling Stone, and the Atlantic published articles about Fetterman before he entered last year's Senate race. More attention followed. He appeared last month on CNN's Parts Unknown, dining and talking about economic revitalization with host Anthony Bourdain. And I hear HBO's Vice News Tonight is about to air a piece about Fetterman and Braddock.

More of that is sure to follow, the kind of "earned" media that candidates embrace to tell their stories. For now, it seems, Stack will continue to lie low and watch it all heading his way.