Pennsylvania’s Democratic Senate primary began for real over the last few days.

For months, Lt. Gov. John Fetterman has loomed over the contest, along with a pivotal question: How firm is the front-runner’s support? Now we’ll start to find out the answer, in a sprint toward the May 17 primary, after a series of attacks by his Democratic rivals.

Fetterman’s opponents have argued — and his Democratic critics have feared — that his consistent lead is mainly due to his relative political fame, and that he would falter once his weaknesses were exposed, either in the primary or against Republicans in the fall. They’ve accused him of trying to mask those flaws by avoiding tough scrutiny.

Starting last week and continuing Sunday during a debate Fetterman skipped, Democratic rivals Conor Lamb and Malcolm Kenyatta began testing that theory with their first full-fledged broadsides against him. They zeroed in on perhaps his biggest vulnerability: a 2013 incident in which Fetterman, then mayor of Braddock, grabbed his shotgun and chased down a Black man after saying he heard gunfire. The man he pointed his gun at was unarmed and had nothing to do with any shooting that day.

Fetterman has defended the armed pursuit as a spontaneous reaction to what he still believes was gunfire, and on Sunday he dismissed the latest attacks against him as desperate. But his rivals argued that it’s a liability Democratic voters need to consider.

“If we are under any illusion that the Republican Party is not going to play an ad over and over again, then we’re not paying attention,” Kenyatta said.

Mike Mikus, a Pittsburgh-area Democratic strategist who ran Katie McGinty’s winning Senate primary campaign against Fetterman in 2016, predicted the race will get closer as more people hear about the jogger incident, particularly Black voters and women. “I just know from my past history and having tested a lot of the hits on Fetterman — I think his support is a mile wide and an inch deep,” Mikus said.

» READ MORE: John Fetterman skipped the first Pa. Senate debate. His rivals made it all about him anyway.

Democrats watching from the sidelines have been waiting for this moment. Fetterman began the race as the favorite and has maintained a 20- to 30-point lead in all public and private polls. The next six weeks will show how much of that is solid support he can hold even as he comes under attack, and as the candidates ramp up TV advertising.

“The race has been going on behind the scenes obviously for a while, but this is the public-facing moment,” said Berwood Yost, a pollster at Franklin & Marshall College.

Until now, the Democrats had jockeyed for endorsements and taken subtle digs at one another, without sustained clashes.

But Lamb and Kenyatta both blasted Fetterman on Sunday for skipping the event and criticized him over the jogger incident. Lamb, who started the criticism on Twitter last week, said Fetterman’s progressive positions will undermine him against Republicans — especially as the GOP tries to label any Democrat a “socialist.”

“That punch is going to land,” said Lamb, a moderate congressman from the Pittsburgh suburbs.

“If John can’t show up to talk to other Democrats about the importance of this race, about what he would do as our next United States senator, who the hell really believes that John is going to be able to show up and stand up and fight for our values in Washington?” said Kenyatta, a state representative from Philadelphia.

The Republican Senate primary has already demonstrated that name recognition doesn’t always translate to lasting support. Celebrity surgeon Mehmet Oz started as the clear GOP front-runner, but polling shows that a flood of attack ads has hurt his standing. Joe Sestak had a consistent lead in the 2016 Democratic Senate primary, until a massive wave of ads hit him in the campaign’s final weeks.

Fetterman has far deeper roots in Pennsylvania politics than Oz does, and likely won’t be as easy to tear down. He’s been a statewide elected official, has a dedicated following locally and nationally, and he’s not going to be overwhelmed by his rivals’ spending: He’s got the most campaign cash by far.

When he spoke to supporters in Chambersburg on Sunday, Fetterman pointed to the 190,000 donors supporting his campaign, many of them small-dollar donors.

“We believe you’re going to need the strongest grassroots campaign possible,” Fetterman told a crowd of about 100. “Just so happens we are the strongest grassroots campaign in this race.”

He also said he’ll attend three televised debates later in the primary. They’ll all come near the end of the contest, when there’s less time for his opponents to take advantage of any stumbles.

In primaries, when the candidates are often not far apart on policy, name ID alone can often be the dominant factor until the final weeks of the race, Yost said. And polling in this race shows a large number of voters still undecided.

» READ MORE: Where the Pennsylvania Democratic Senate candidates do and don’t agree

But Fetterman holds some major advantages.

An Emerson College survey released last week found about a third of Democratic voters undecided, but about half of those undecideds lean toward Fetterman, with few considering Lamb, according to Emerson pollster Spencer Kimball.

“Fetterman’s in a strong position at this point,” said Kimball, whose survey showed Fetterman with 33% support in the Democratic primary, 23 points ahead of Lamb.

Even if Lamb lands a punch on Fetterman, going negative can also bring down the attacker. And if he peels off some of Fetterman’s supporters, Kenyatta could also scoop them up.

“Anytime you go negative, you throw mud at somebody, we still have a little bit of mud on ourselves, so there’s that risk for Lamb,” Kimball said.

Perhaps most important, according to Democratic operatives, is that Fetterman has such a cash advantage he could swamp attempts to bring him down with his own positive message.

“If you’re dealing with someone who’s really well-known, it’s a harder task to change perspectives,” Yost said.

He noted that Fetterman has a strong social media presence that has helped build loyalty among supporters.

“The longer he stays at a third of the vote,” Kimball said, " the more it will crystallize.”