In a state that pollsters and analysts call tailor-made for Hillary Clinton, Obama is seriously challenging the fit.

So much so, some wonder if an unraveling's in the offing.

One could make a case.

The average of multiple new polls, including one putting him slightly ahead, shows he has trimmed a large Clinton lead - 20 points in January, 16 last month - down to single digits, at 5.4.

His campaign yesterday announced that it raised $40 million in March amid persistent chatter that Hillary's running low and reports that she raised just half that.

More money means more TV, and Obama's clearly clubbing Clinton.

Last week alone, he spent $2.2 million on Pa. TV ads, compared with her $627,000.

Then he waged a ground war.

He crisscrossed the state on a bus tour with stops in places that should be hers or the GOP's.

In the process, he picked up six days of free and almost entirely positive press.

He went to blue-collar Johnstown, home of big-name Hillary backer U.S. Rep. John Murtha.

He went to hardscrabble Scranton, birth and burial place of Hillary's father.

He went to rural Lancaster County, home of the state's most conservative Republican voters.

And even though he didn't eat a cheesesteak in Philly, well, neither did she.

Point is, he's running hard in a state some said he'd write off since it's so, well, Clinton's - older than the national average, whiter than the national average, less educated than the national average and full of Catholics and union members.

It's also a state where much of the Democratic establishment, including Gov. Rendell and Mayor Nutter, endorses Clinton.

Yet her poll numbers fall.

Not that that's unusual.

Before the most recent big-state primaries in Ohio and Texas on March 4, the same thing happened.

Clinton led in Ohio by 26 points in December, 23 in January and 14 in February. She won the state by 10.

In Texas, she was up 16 in mid-February, 6 in early March and won the state by 3.

And this was as bad things happened to Obama: An economic adviser allegedly told Canada that Obama's anti-NAFTA talk was just politics, and Clinton's first "red phone" ad seemed to cost him votes.

But in Pa. (so far), bad things have happened to Clinton.

Just days before Obama's bus tour, Clinton told the Daily News editorial board that she "misspoke" about landing in Bosnia in '96 under sniper fire.

The story went nuclear, costing her some credibility and helping soften aftershocks of Obama's Rev. Wright.

Then, just as Obama's tour was starting, U.S. Sen. Bob Casey Jr. - not a man known for surprises - surprised many by endorsing Obama at precisely the moment to do him the most good.

It allowed Obama to reach out to Pa. blue-collar, socially conservative Democrats while carrying the seal of approval from a brand-name pol whose core constituency is key to victory and the same one Obama seeks.

Finally, the usually somnambulant Pa. electorate is interested.

New voter-registration figures show a record four million-plus Democrats, including more than 234,000 new Democrats, more than half of whom switched parties to vote on April 22.

This is clearly a plus for Obama. He, not she, brings in new people.

Who knows what happens next week or the week after? Elections, as I've written, often depend on what windows open or close and when.

A misstep or disclosure at a critical time can make all the difference.

All I'm saying is that Obama had a pretty good week - good enough to wonder whether he wins Pa. *

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