The polls for the Pennsylvania primary (now only two weeks away) have been mixed, but overall show a tightening of the race. Is this a function of Penn-gate and Sniper-gate or of Barack Obama’s unprecedented ad buy? Perhaps both.

On Hillary Clinton’s side of the ledger is her “don’t let them chase me out of the race” appeal and her persistent efforts to undermine Obama’s image as a credible commander-in-chief. And, of course, there lurks the hope that her lead is still commanding and the polls haven’t really tightened, that voters have just stopped being honest with pollsters. (This is the last refuge of a losing campaign– a refusal to believe all the available polling.)

The race will effectively end if Clinton loses. Even she would be hard pressed to continue if she loses one of the “big states.” To “count,” remember, a state in Hillary-think has to be a primary state, a big state, a state the Democrats could reasonably hope to win (Texas still counts, though), and one she is not “expected” to lose. Other than excluding states beginning with the letter “P,” it would be hard to rationalize her continued candidacy after a Pennsylvania loss.

Barring that outcome, or, almost as unlikely, a blowout in her favor, both sides will play the expectations game. Obama will say, “Oh my, I’m the comeback kid and only lost by 10 pts. Hillary is through!” Clinton will say, “I was outspent and still trounced him by 10 pts.” Then everyone will trudge on to Indiana (where she leads) and North Carolina (where he does).

So if you think we are nearing the end of the Democratic primary, think again. We may have months more to go.