Two hundred years ago, just before the Iowa caucuses, the Jeffersonians referred to Pennsylvania as “the keystone in the Democratic arch.”

This week, Hillary Clinton got to make the case she needs to stay alive in this presidential race:

How do Democrats build a majority in November without their keystone?

And wouldn’t it also be useful to have a candidate with a better chance of carrying Ohio, New Jersey and Florida?

Clinton’s victory Tuesday, by 10 points, enables her to claim that she offers the party’s best chance against John McCain in November, because the states that have decided the presidency over recent decades tend to prefer her. Whether that causes superdelegates to pause and reconsider is hard to know, but Clinton at least has an opening to make her case.

The case is, as her campaign chairman, Terry McAuliffe, exulted on CNN Tuesday night, that she is “able to win one of the key Democratic states that we need to win in November.”

Clinton also demonstrated, once again, that she is hard to put away.

For the fourth time – New Hampshire, Super Tuesday, Ohio-Texas Tuesday, and now in Pennsylvania – Obama was unable, as Clinton says, “to close the deal.” In Pennsylvania, Obama spent $11.2 million on media – more than in all of Super Tuesday, enough to appear on Philadelphia TV more often than the Phillies – against her $4.4 million, and lost ground in the closing days.

According to exit polls, Pennsylvanians who decided during the last week went for Clinton 58 percent to 42 percent – a pattern familiar from Ohio, California and New Hampshire.

“I might stumble, and I might get knocked down,” Clinton told her victory rally, “but as long as you’ll stand with me, I will always get back up.”

She also ran strongly among the groups consistently identified as swing voters: Catholics (68 percent); blue collar voters; and, in the first primary campaign between a woman and an African-American man, a new swing group – white males, who apparently went for her by 10 percent.

Clinton wasn’t the only winner in Pennsylvania. Obama added to his delegate total, getting closer to the number where he’ll seem unstoppable. And for all the breathless cable chatter about how “incredibly negative” the campaign was, Pennsylvania still produced heartening numbers for Democrats in general.

For a long time after the polls officially closed in Pennsylvania Tuesday, there was only one clear, unquestionable number showing its face in public:

An awful lot of people voted in the Democratic primary.

In fact, the primary had a record turnout, breaking the total from 1980. (In most states, that wouldn’t be remarkable, but Pennsylvania hasn’t exactly been growing.) One voter in seven, according to exit polling, had not been a registered Democrat at the start of the year.

The sharpest division among primary voters was 47 percent to 42 percent – between voters who thought the country was in a moderate recession and those who thought it was a severe recession.

For Republicans, that’s not a promising division.

Half the voters said they connected with both candidates, and while Clinton led Obama, 27 percent to 14 percent, on the question of who would improve the economy, by far the largest number – 47 percent – said that either of them would.

On the other side, only 73 percent of voters in the Republican primary voted for John McCain.

“Democrats should be celebrating this,” Clinton supporter Paul Begala said of the Obama-Clinton contest, “because wherever the two of them go, Democrats win.”

Or at least, more Democrats show up.

From Philadelphia through the coal country to Pittsburgh, more of them did, and kept the Democratic primary campaign going.

With Pennsylvania, the Jeffersonians assured themselves, they could stand against the universe.

With Pennsylvania, Hillary Clinton can stay alive.

And raise some questions.

David Sarasohn is an associate editor at The Oregonian of Portland, Ore. E-mail [email protected].