A few points:
1. How bad was the contest for the D nomination? Is it over after Iowa, NH, and maybe one quick one. Kerry was basically the nominee by default; nobody could stomach the others. Dean suddenly gains appeal within the party, and he wins by the end of February, he's much stronger in the fall.
2. Dean, not having to move to the left so much in the Primaries, emphasises his moderate side:
A. Gun owning rights become a cornerstone of his campaign.
B. He treats abortion as a "medical issue" and follows Bill Clinton's "safe, legal, and rare."
C. He favors "contracts" between two people of the same sex, "What two adult people do in the privacy of their own home, so long as they are hurting anyone, isn't government business," and but opposes marriage, noting that is more a religious concept and should be left to churches. He addresses some subsidiary issues, like pensions and divorce for same sex contracts.
By this point in time, he had already set himself up as the Liberal candidate. He would have had a hard time abandoning his positions from the early primary days once attack ads were running again. He would have walked right into the flip-flop charge.
