I would say continuation of the Democratic primaries, where no clear front runner has emerged, hurts their chances of winning in the November general election.
First, there can only be so much mud and insults thrown around before people become turned off and look at the alternative candidates.
Second, we all know the key to winning an election is money. Each day the Democratic primaries are drawn out, that's more dollars spent by the candidates. Sure, they keep getting contributions, but will people necessarily contribute a second time in the general election? A long primary process drains Democratic financial resources in just the nomination process.
Less time is given for national campaigning as well. John McCain is already working on his national campaign strategy while Clinton and Obama try to make the other look as bad as possible so they can get their party's nomination. I think the Republicans will have an advantage going into the general election.