The Democrats assume that independents won't find the GOP an attractive vehicle for their disaffection, while Republicans assume that independents will unite against the Democratic Party's agenda in Congress: health care reform, cap-and-trade, the stimulus package, liberal social policy legislation. That means that Republicans will spend 2010 attacking the Democratic Congress, while Democrats will spend 2010 attacking the Republican brand -- reminding independents why they disassociated with the party in the first place.
A political assumption among Democrats is that health care reform will be popular in the short run -- and when the implementation phase begins, it may be unpopular again -- but it will, in the long-term, be a bragging point for the party.
A political assumption among Republicans is that Democrats will get no credit for health care reform.
A patronizing assumption among Democrats is that Republican Party leaders will fail to find attractive, moderate candidates and run them in appropriate districts in 2010.
A patronizing assumption among Republicans is that Nancy Pelosi isn't smart enough to figure out how to give centrist Democrats leeway to buck the party in 2010.
A corollary assumption for Democrats is that Republicans cannot build a national movement.
A corollary assumption for Republicans is that Democrats consistently underestimate the long-term brand failure of liberalism.
An unspoken assumption among Democrats is that the economy has put people into a funk. The quicker the economy improves, the more attenuated the incipient populism will be. (You'll never hear Democrats say this, shades of Jimmy Carter's "malaise" speech, a word, of course, he never actually uttered.)
An unspoken assumption among Republicans is that cultural politics, dampened by the failure of the Bush presidency and the collapse of the economy last year, are back and tilting the electorate decidedly. Broadly defined, this includes fears about multiculturalism, political correctness, gay rights, and immigration.