What's the Difference Between A Promise & a Pledge?

Democrat Dan Onorato, who’s criticized Republican opponent Tom Corbett for signing a “no new taxes” pledge, is now promising not to increase Pennsylvania’s revenues.
For months now, Onorato has dismissed Corbett’s pledge not to raise taxes as a “gimmick.”
Campaigning in Harrisburg, Onorato promised he wouldn't raise taxes...
"I’m not raising the revenue. I’m going to live within our budget. I can’t tell you what year 7, 8, 9, 10 down the road…I’m talking about, the next governor’s going to come in with anywhere from two to four billion in deficits, depending on whose numbers you’re using. And I’m telling you, you don’t fix that by raising revenues. We’re going to come in and live within our means of what we have."
The promise does not include a Marcellus Shale severance tax.

So why is a pledge a gimmick, but a promise sincere?

"Because I’ve never signed pledges. I’ve been elected for a long time, I’ve never – I’ve always thought when people at a debate throw you something to sign on any kind of pledge – to me, that’s just politics."

Corbett’s spokesman, Kevin Harley, says Onorato is avoiding the pledge
"If Dan Onorato is so committed to not raising taxes, then he should join Tom Corbett and sign a “no new tax” pledge. But he refuses to. Why does he refuse to? Because he’s going to raise taxes."

Corbett generated headlines last week, when he backtracked on a previous statement, and said his pledge doesn’t cover increases in state fees.