Hillary Clinton declared the "tide is turning" Tuesday after scoring a critical victory in the Pennsylvania Democratic primary, pushing the race ever forward to the nine remaining contests.

With 99 percent of precincts reporting, Clinton had 55 percent and Barack Obama had 45 percent, a comfortable enough margin to deny critics their demand that she quit the race.

"Some counted me out and said to drop out," Clinton told cheering supporters at a rally in Philadelphia. "But the American people don't quit. And they deserve a president who doesn't quit either."

Having already locked down the Republican nomination after the March 4 primaries, John McCain easily coasted to victory in Pennsylvania Tuesday, pulling 73 percent with 98 percent of precincts reporting.

Clinton beat Obama in the Keystone State primary with a big boost from her core constituencies, including lower income and union households, but she also ended up overperforming among some groups that are Obama strongholds, including college-educated voters and last-minute decision makers.

Pre-election surveys cast Clinton as the favorite in Pennsylvania, but she was under pressure to win big in the state. Trailing Obama in pledged delegates, she's trying to win over uncommitted superdelegates by arguing that she's more electable against McCain in a general election. A big win in Pennsylvania helps strengthen her argument that she is dominating in large swing states critical to Democrats in November.

"Hillary has won all the states we have to win in the general election," Clinton campaign chairman Terry McAuliffe told FOX News after the race was called, touting the fact that Clinton won Pennsylvania despite being largely outspent in the state by Obama.

Clinton, too, stressed that she did not have the same cash flow as Obama and urged supporters to donate to the campaign. The Clinton campaign claimed it raised $500,000 online in the first hour after polls closed Tuesday.

Obama said Tuesday night that he "closed the gap" in Pennsylvania, despite those who "didn't think we could make this a race."

One hundred fifty-eight delegates are at stake in Pennsylvania, the largest payoff of any contest left on the calendar.

But Indiana and North Carolina together offer more than that when those states vote May 6, and Obama was already gearing his campaign toward Indiana as results from Pennsylvania rolled in. He held his election night rally in Evansville, Ind., while Clinton held hers in Philadelphia.

Speaking in Indiana, Obama offered a warning to voters not to be distracted by political bickering -- the kind that may have hurt his performance in Pennsylvania.

"After 14 long months, it's easy to forget what this campaign is about from time to time, to lose sight of the fierce urgency of this moment. It's easy to get caught up in the distractions and the silliness and the tit-for-tat that consumes our politics -- the bickering that none of us are entirely immune to and that trivializes the profound issues: two wars, an economy in recession, a planet in peril," he said.

Exit polls in Pennsylvania showed Clinton hanging on to key voting blocs -- women, seniors, whites, union members and lower-income households. The polls showed she was leading in union households by 57 to 42 percent, and among seniors by 60 to 39 percent.

But Obama was holding a commanding lead among black and young voters, as well as among college-educated voters, who exit polls showed were breaking for Obama by a margin of 54 to 46 points. Among urban voters, he was getting 69 percent of them compared with 31 percent for Clinton.

Robert Friedrich, political science professor at Franklin & Marshall College in Lancaster, Pa., said earlier Tuesday that a big margin -- of at least eight or nine points -- was critical for Clinton in Pennsylvania.

But Clinton said Tuesday that "a win is a win" and early on questioned why Obama was unable to turn things around.

"I think maybe the question ought to be, 'Why can't he close the deal with his extraordinary financial advantage?' Why can't he win ... this one if that's the way it turns out?" Clinton said Tuesday.

As polls showed Clinton consistently ahead in the state, Obama downplayed expectations going into the race's final hours, and his campaign argued that the outcome of the primary will not change the dynamic of the race.

"It's an uphill battle," Obama said, campaigning at a Pittsburgh diner earlier in the day.

An Obama campaign memo claimed, "The Clinton campaign needs a blowout victory in Pennsylvania to get any closer to winning."

Privately, the Clinton campaign was shooting for the big win. A large margin would help her make a significant dent in the delegate count, and help her convince uncommitted superdelegates to back her candidacy.

As of Tuesday evening, Obama claimed 1,705 delegates to Clinton's 1,575. They're aiming to reach 2,025 delegates to clinch the nomination.

The Pennsylvania vote came after an increasingly bitter and negative contest between the two Democratic candidates.

In the closing days of the campaign, Obama cast doubts on his rival's honesty while Clinton questioned whether Obama was thick-skinned enough to handle the pressures of the presidency. She hammered him for telling a group of California donors recently that small-town voters "cling" to religion and guns out of bitterness over lost jobs.

Exit polls, however, showed that voters thought Clinton was the one hitting below the belt the most.

Asked which side was fighting unfairly, 68 percent said Clinton and just under 50 percent said Obama.

Meanwhile, near-record turnout was the expectation in Pennsylvania, similar to other states that have seen voters flood the polls.

Pedro A. Cortes, Pennsylvania secretary of state, said he expected turnout to be between 40 and 50 percent. Turnout exceeded 2.25 million Democratic voters.

"This feels like a general election, not a primary," he said.

About one-third that many Republicans went to the polls in the mostly ceremonial GOP vote.

FOX News' Aaron Bruns, Bonney Kapp, Caroline Shively and Judson Berger and The Associated Press contributed to this report.