JUST HOW LOW can Mayor Nutter go on May 17?

We're talking about the mayor's primary-election showdown with former state legislator and ex-offender Milton Street. And although nobody seriously thinks Nutter can lose the race, he certainly could lose some face.

The better Street does, the worse Nutter looks.

"There's a point at which people interpret it as a vote of dissatisfaction, especially from the African-American community and aimed at the mayor," said Zack Stalberg, president of the watchdog group Committee of Seventy.

Sure, Street is a clown-like figure in many ways, a sideshow act. The brother of former Mayor John Street recently got out of prison for not paying his taxes and still owes more than a million bucks in back taxes to the IRS, the state of New Jersey and the city.

Four years ago, while running for City Council, he stood next to a coffin on a stage outside City Hall and serenaded Philadelphia's Center City lunch crowd.

But his windmill-tilting campaign has already had some surprising successes. The city's firefighters' and blue-collar workers' unions recently endorsed Street in what can only be interpreted as a defiant nose-thumbing at Nutter, who has squabbled with them over contracts.

After years of city cutbacks because of national economic decline and recent hikes in property and sales taxes, many voters are restless. So it does appear that there will be a protest vote out there.

"Milton Street is silly and he is who he is. And no one wants him to be the mayor, but that doesn't mean people won't vote for him because they're mad at the world," said campaign consultant Neil Oxman, who worked on Nutter's 2007 race and is involved again this year.

Nutter said that he knew the city's budget woes had taken their toll, but he stressed that he was busy talking to voters about a brighter future for the city. As for Street's shenanigans, Nutter said it was a free country.

"In a campaign, the one thing I have some ability to control is what I do," Nutter said. "I think it's also clear that the vast majority of those who have made an endorsement have endorsed my re-election."

Still, oddball characters like Street - who did not return a call for comment - have gone far in politics before. Take, for example, Christine O'Donnell, who won the Republican U.S. Senate primary in Delaware last year, defeating a nine-term U.S. representative.

O'Donnell won despite a slew of negative reports about personal financial problems and fudging her educational records. And who can forget the old television appearance during which she claimed to have dabbled in witchcraft as a teenager.

In the end, though, even magic powers couldn't win O'Donnell the general election.

The big question is just how big the anti-Nutter vote will be. Experts agreed that Street could easily hit between 5 and 10 percent just with your average contrarians. But if that vote creeps much above 10 percent, it likely suggests some anger. And if it gets close to or above 20 percent, that's a real sign that voters are mad at Nutter.

"Once you get over 10, you have more than the contrarians and the crazy," said political consultant Ken Smukler. "When you get over 15 and into the 20s, then you have an opposition. It means that part of your base has decided they will go with anyone over you."

If the number climbs high enough, there are two possible fallouts for Nutter. In the short term, a strong showing by Street could prompt an independent candidate to consider opposing Nutter in the general election.

"If Nutter becomes that weak, relative to Milton Street, people can say, 'I can get AFSCME, I can get the firefighters,' " Smukler said. "It's not that difficult once Milton gets into the 20s for an individual to say, 'I can get my 1,800 [signatures to get on the ballot as an independent.]' "

Another outcome of a high turnout for Street would be the diminishment of Nutter's clout and influence in his second term. That's always a risk anyway as a mayoral career winds down, but Oxman cautioned that Nutter should still have plenty of ammunition to get the job done.

"It's still a strong-mayor form of government; he's still the guy who gives out the contracts," said Oxman.