After a six-week hiatus, the Democratic presidential contest goes back to the voters tomorrow when Pennsylvania holds a primary that is a make-or-break contest for Hillary Rodham Clinton's struggling campaign.

It also is a test of whether Barack Obama can regain his momentum despite a cascade of recent controversies.

The last few weeks of campaigning have not been kind to either candidate: Clinton's once-commanding lead in Pennsylvania polls has dwindled, while Obama has come under fire for his former pastor's incendiary remarks, his own association with a Vietnam-era radical and a comment that seemed to demean "bitter" rural voters.

The stakes in Pennsylvania are high as the candidates head into the stretch drive of a race being run on two levels.

On one level, they are still fighting to collect more committed delegates to the nominating convention, although it is hard to see, mathematically, how Clinton can overcome Obama's advantage. But because, in this close contest, neither can sew up enough elected delegates to clinch the nomination, they also are campaigning for the hearts and minds of "superdelegates" — the party activists and officials who can back either candidate and will likely decide the contest.

The support of 2,025 convention delegates is needed to win the nomination. According to a count by The Associated Press over the weekend, Obama had 1,645, compared with Clinton's 1,507, even though she had the backing of more superdelegates, 258-232. Obama is ahead in the popular vote as well, 13.4 million to 12.7 million.

Pennsylvania has 158 delegates up for grabs, and the state's demographic makeup plays to Clinton's strengths. But if the New York senator does not score a decisive victory to close the gap in both the popular vote and the delegate count, her hopes for muscling her way to the nomination will fade, if not vanish.

Obama has flooded Pennsylvania with campaign money and spent weeks in the state. But if the Illinois senator proves unable to reach beyond his well-established coalition of black, affluent and well-educated voters, it may raise questions among uncommitted superdelegates about his strength as a candidate in the general election.

Blue-collar white voters will be a crucial constituency in the campaign against John McCain, the presumptive GOP nominee.

A BLANKET OF ADS

Two days before the Pennsylvania primary, Obama and Clinton observed the rituals of Sunday campaigning: a visit to church, stops at restaurants catering to families, as many public events as possible.

And blanketing the state with attack ads.

"In the last 10 years Barack Obama has taken almost $2 million from lobbyists, corporations and PACs. The head of his New Hampshire campaign is a drug company lobbyist, in Indiana an energy lobbyist, a casino lobbyist in Nevada," said a new Clinton commercial airing in the campaign's final days.

If anything, Obama upped the ante with his rebuttal. His ad said he "doesn't take money from special interest PACs or Washington lobbyists — not one dime." Clinton does, it added, and accused her of "eleventh-hour smears paid for by lobbyist money ... "

After the breakneck pace of the first three months of the 2008 primary season, the six-week run-up to tomorrow's primary has been the longest stretch of pure campaigning since the process began in January. Pennsylvania is the first of 10 states and territories that have contests scheduled before Montana and South Dakota close out the voting calendar June 3.

A recent Los Angeles Times /Bloomberg poll showed Clinton leading Obama in Pennsylvania by 5 percentage points, a decline from previous polls that had her ahead by double-digit margins.

The state has been considered hers to lose because it is packed with the constituencies she has dominated — the elderly, Catholics, blue-collar whites — especially in rural areas and in western Pennsylvania. She has been endorsed by much of the state's Democratic establishment, including the popular governor, Edward G. Rendell, and Philadelphia Mayor Michael Nutter.

"She obviously has a home-court advantage," Rep. Chaka Fattah, D-Pa., an Obama supporter, said yesterday on CNN's "Late Edition."

But Obama has high-wattage endorsements of his own, including that of his Senate colleague, Bob Casey, an anti-abortion Democrat who is expected to ease conservatives' concerns over Obama's support for abortion rights.

MARGIN SEEN AS CRUCIAL

With Clinton still favored to win, much attention will focus on the margin of victory. The most decisive outcome would be an upset victory by Obama — a likely death knell for Clinton's candidacy. Some analysts argue that Clinton needs to win the state by double-digit margins, as she did neighboring New Jersey and Ohio, which she won by 10 percentage points. But her supporters have been downplaying expectations.

"Would I like (Clinton) to win by double digits?" Rendell said on CBS' "Face the Nation." "Sure, but I don't think that is going to happen."

Clinton's campaign aides point to the fact that Obama has outspent her 3-to-1 in Pennsylvania, breaking records for the amount of money spent on a statewide race.

"They have created the best possible playing field for themselves," Clinton spokesman Phil Singer said. "If they are unable to claim an outright victory after all of that, it will be yet another example of Sen. Obama failing to win a key state that will be needed in the general election."

The Associated Press contributed to this report.