MAYOR NUTTER is vulnerable.

More than half the city wants to deny his bid for re-election this year, a recent poll shows. City Council routinely upends his agenda. Ward leaders are angry over how he runs the city.

So, big-name Democrats and Republicans must be eagerly lining up to take Nutter down, right?

Nope.

It's been nearly a quarter-century since an incumbent mayor of Philadelphia faced a serious primary-election challenge, when Ed Rendell could not unseat then-Mayor W. Wilson Goode Sr. in 1987.

Tom Knox, a millionaire who spent $12 million and finished second to Nutter in the 2007 Democratic primary, yesterday said he would not seek a rematch.

Knox, who for months has said that Nutter is doing a bad job, then cheerfully endorsed the mayor.

Sam Katz, who ran three times for mayor as a Republican and considered challenging Nutter this year as a Democrat, said political parties in the city lack competition and credible candidates.

"The Democratic Party in Philadelphia has lost its way," Katz said. "It used to be a very competitive place."

As for the Republicans, Katz said there was little GOP help for the 2007 nominee for mayor, Al Taubenberger, and no reason to expect any better for a candidate running this year.

"The Republican Party, to its lack of credit, doesn't think past the next election when it comes to developing candidates," he said.

Nutter's only challenger in the Democratic primary election is T. Milton Street, a master or jester of political street theater - depending on whom you ask - who just got out of federal prison.

The only Republican candidate, real-estate agent John Featherman, is a four-time loser in campaigns for public office and is opposed even by local GOP leaders.

The deadline to run for mayor in the May 17 primary election is now less than two weeks away.

G. Terry Madonna, director of the Franklin & Marshall College Poll, said a candidate challenging Nutter faces huge costs in campaign resources.

The reward for a victory would be a city with a troubled economy, facing dwindling state and federal resources.

With several Council seats in play this year, the "political energy" has been focused there, Madonna added.

Featherman said Nutter is getting a free pass from Democrats who would rather wait for the big fight for an open seat in 2015. Republican leaders, Featherman said, have been stymied in their search for a candidate to endorse because they can't promise an uncontested primary race.

Nutter's political problems apparently won't matter this year:

_ The Franklin & Marshall poll showed that 53 percent don't think Nutter should be elected, with the same amount saying that the city is heading in the wrong direction. Nutter's job-approval ratings stood at 50 percent, dropping to just 42 percent among blacks and soaring to 60 percent among whites.

_ Nutter, who heads the city's 52nd Ward, met Saturday with the Democratic Ward Leaders of Color to discuss their endorsement this year. Despite being a member of the group, Nutter has to wait for a vote on the issue.

_ Nutter, who ran in 2007 pledging to reform city government, has been the subject of former Mayor John Street's ire for months. Street is still trying to recruit a candidate to challenge Nutter after striking out with Katz and Knox, though he does not appear to have any role in his brother Milton's campaign.

But Nutter also has a few things going for him this year:

_ He laid an early bet on campaign-finance reform during his time on Council that is now paying off politically. A candidate who takes him on would be limited to taking $2,600 from individuals and $10,600 from political-action committees this year because of the city's campaign-finance law, making it difficult to raise big money.

_ Nutter had $1.6 million in his campaign account at the end of January.

_ Nutter's two biggest threats in the primary - state Sen. Anthony Hardy Williams and Councilman Bill Green - never looked serious about running. The poll showed that one in four held favorable views of both men but that a large majority were undecided or had not heard of them.