Gore would have been able to avoid the "I voted for it before I voted against it" gaffe.
But W always played dirty - swiftboat ads in OTL 2004, the black baby push poll in the 2000 primary, etc. He always had third parties do it for plausible deniability, but by some cosmic coincidence those people always showed up when he ran and always targeted his opponent. So it's safe to assume some type of attack on Gore would gave happened.
Gore was also every bit as wooden and uncharismatic as Kerry, and was more focused on issues important to urban white hipsters than on economic populism. Which means that unlike John Edwards, he wouldn't have moved the needle much compared to Kerry. Even Gephardt might have done better (not on persona but on substance).
One other factor in 2004: That was the year the Massachusetts Supreme Court legalized SSM. Karl Rove engineered the state constitutional amendments banning SSM. Those ballot initiatives caused Moral Majority types to show up at the polls in greater numbersnumbers. Some of those voters were lukewarm toward Bush and would have stayed home, but when they came to the precinct to vote for something else, they figured they might as well vote for W while they were there. Any D nominee would have been affected by that. You would have needed a strong enough opponent to overcome those extra R votes and Gore wasn't it.