During the 2004 election cycle Mitt was still considering running for reelection in '06, so he maintained his liberal positions on social issues and invested in the state GOP.
During the 2006 election cycle Mitt had decided to run for president and started adopting a new position on abortion, finding meaningless symbolic gestures to pander to conservatives nationally and get himself headlines, invested money nationally to win himself support in the primaries, and abandoned the Massachusetts GOP and started making fun of the state on his out-of-state campaign stops.
The guy's a weasel, a Clinton clone from the Bay state. (Remember, Clinton did a similar switch from pro-life to pro-abort when he was preparing to go national.)
Slick Willard for ya.