We have reached a milestone in the Democratic presidential race. Two numbers tell the story:

217 and 257.

There are only 217 delegates left to be chosen in all the remaining primaries. Their number is now vastly exceeded by the 257 superdelegates who have yet to choose on their own.

Here are two more numbers to consider:

178 and 329.

Barack Obama needs only 178 more delegates to clinch the nomination. Hillary Clinton needs 329.

In other words, the 257 fence-sitting superdelegates could end this race right now if only 70 percent of them announced for Obama. That might sound like a daunting share, but it's actually smaller than the percentage that Obama has posted since Tsunami Tuesday on Feb. 5. Between that date and the middle of April, Obama won the allegience of 93 superdelegates; Clinton, only five.

This week alone, Obama has netted at least eight new ones, Clinton only one. Her once-daunting advantage among superdelegates - attributable to her longstanding insider status, and that of her husband's - has been whittled away nearly to nothing. She once lead by several hundred; according to CNN, that lead now stands at eight, and NBC puts her lead at nine. (Saturday update: ABC and The New York Times now report that Obama, for the first time, is tallying more superdelegates than his near-vanquished rival.)

So, Obama fans may well be asking, what's the holdup? Why don't the superdelegates stick a fork in this race and call it over?

The answer requires one more number: 90. That's the share of unpledged superdelegates who make their living in Washington, as elected politicians - almost half of the superdelegate pool. And they're basically holding out for the simple reason that, if they jumped now for Obama, a lot of their Hillary-friendly constituents would be seriously ticked off, perhaps enough to retaliate against those politicians when they run for re-election this fall. And many represent conservative and/or rural constituencies.

One example: Senator Mary Landrieu of red-state Louisiana. She faces a tough re-election this fall, and doesn't want to alienate any Democratic voters, given the fact that she needs them all. She told MSNBC on Wednesday that she intends to stay neutral, "out of respect for my supporters, half of whom are for Senator Clinton and half of whom are for Senator Obama."

All this suggests that grassroots Clinton followers - taking their cues from the candidate - are not yet willing to concede this race, to countenance any surrender. The period of reconciliation has yet to begin, and it won't happen until Hillary Clinton gives the signal. Presumably, at some point, she will - when the math becomes inescapable.

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My conversation with Senate Majority Leader Harry Reid, which took place earlier today at Philadelphia's Free Library, is available in a podcast, on the library website. He voiced disappointment that the Senate Democrats haven't been able to do more to stop the war. He also said that the Clinton-Obama battle has been great for the party. I tried to pin him down on a few pressing political matters, but he deftly bobbed and weaved like the amateur boxer he once was.