Nutter, a reform-minded former city councilman who took office in January, endorsed Clinton in December while she was the front-runner.
Since then Obama's bid to become the first black president has garnered more votes, more delegates and more donations than the New York senator's equally historic bid to become the first female president.
Revived by March 4 wins in Ohio, Texas and Rhode Island, she is now favored to win Pennsylvania, in part because overall the state is demographically so much like neighboring Ohio. But given Obama's overwhelming support so far from blacks, there is little doubt that the Illinois senator will prevail among Philadelphia's 1.4 million residents and its nearly equal numbers of black and white voters.
Nutter's reaction: "This notion that somehow there is a monolithic black vote is just a myth."
He has promised to campaign aggressively for Clinton "just like I campaign for myself," doing events and raising money. He said he supports her policies and thinks she has the best chance to win in November. "Our best matchup is Clinton-McCain," he said, referring to Arizona Sen. John McCain, who has locked up the Republican presidential nomination.
In a tight contest where every delegate has become important, Nutter's backing could prove very helpful for Clinton if he can help hold down Obama's margin of victory in Pennsylvania's largest city. In addition to Nutter, she also has the backing of former two-term mayor, Gov. Ed Rendell.
On April 22, Pennsylvania will apportion 158 delegates, the biggest remaining prize, among candidates based on their relative support.
As Obama was stringing together 12 wins in late February and early March, some black Clinton supporters came under pressure to switch sides. Late last month, her most prominent black supporter, U.S. Rep. John Lewis of Georgia, a hero of the 1960s civil rights movement, switched his endorsement from Clinton to Obama in what he called one of the toughest decisions of his life.
Nutter said he has felt no pressure to switch and has not considered it.
"People elected me to bring about change," said Nutter, who is on the ballot as a Clinton delegate. "There is less concern about race than there is about results."
Like Clinton, whose campaign experienced several near-death moments, the 50-year-old mayor also staged a comeback. The Wharton School graduate prevailed over four other candidates in last May's Democratic mayoral primary after trailing badly in the polls for months.
The mayor was elected in a landslide last fall by a broad cross-section of black and white voters looking to him to end corruption in city government and reduce the homicide rate, in part by giving police more latitude in conducting "stop and frisk" searches.
City Councilman W. Wilson Goode Jr., who is supporting Obama, said he has no reason to believe Nutter will suffer any voter backlash for his Clinton endorsement — partly because of timing.
"Mayor Nutter's re-election campaign is at least three years from now, and I believe he will be judged on his record," Goode said. "He is too early into his tenure for any one thing to destroy or impact his chances of re-election."
In the 1984 Democratic presidential nominating contest, Goode's father, the city's first black mayor, endorsed Minnesota Sen. Walter Mondale over civil rights leader Jesse Jackson.
Likewise, Nutter said he isn't particularly concerned about turning off any of his supporters by endorsing Clinton.
"There's a risk every day of alienating somebody," he said. "No one has said anything to me."
But Cam Mason, a 62-year-old who works in finance and had stopped by a lunch cart in downtown Philadelphia, said that although Nutter is entitled to his opinion, he should think of his constituency before making an endorsement.
"Rallying support of a candidate is sort of tantamount to saying that this is the way he feels the people feel," Mason said. "And I don't know if that's the case."
This year, 12 of the 14 Democrats on the City Council have so far chosen sides, with six for Clinton and six for Obama.
Statewide, Clinton has held an advantage over Obama in independent polls, though Obama has chipped away at her lead. She is expected to do well in blue-collar southwestern Pennsylvania, just as she did in neighboring Ohio, and in northeastern Pennsylvania, which includes the city of Scranton — where her father was born and she was baptized.
