...although given the pathetic state of the Democratic party Republicans actually might not lose. There again, it's not much of a win to beat many Democrats.
Here is my guess, Republicans will lose because they've abandoned their core consituency on things like immigration, economics and so on. I.e. the policies they support are double-minded and hypocritical and what is said doesn't match what actually happens. Examples: They pass tax cuts but cannot stop spending or veto anything, they put the national guard on the border but cannot stop illegal immigration, etc. They aren't actually doing what their core constituency wants them to do. In general, they are failing to represent the views of people who get Republicans elected. Therefore, they probably won't be elected again unless Democrats are really, really stupid. (Which is a possibility, e.g. putting Howard Dean in charge, the election of Ned Lamont, etc.)
Ironically, if my guess is right and they do lose Democrats will probably read the elections results as a referrendum on Iraq. Like their position on tax cuts where they cannot see the capacity that people have for behavioral adaptaion the Democrats believe in myopic type of "scientific" worldview where billiard ball type cause and effect determines outcomes. They view people and organisms as bound and determined by their history* and so do not focus on the capacity for dynamically adapting that is actually typical to both organisms and people.
So they won't see it coming.
[Edit: I hope to revise and finish this later. Once again...]
To continue, they wouldn't see their defeat coming. If Republicans lose and that causes the Old Press and American Leftists to believe that they succeeded in what they've been trying to do over the past years then more and more Leftists will take over the Democratic party, leading the Democratic party to go against American militarism, leading it further into the electoral defeat that it has come to know and hate under Karl Rove. Some seem to think that Rove is some sort of malevolent genius but it doesn't take much of a genius of any sort to see that going against American militarism is politically stupid. Yet there is John Kerry, then there is Murtha, etc.
Given their pseudo-Newtonian worldview, the view of many Democrats seems to be that billiard ball A (the American public tiring of the war) will hit billiard ball B into motion (a sufficient cause for the election of Democrats) and that's the end of it because everything is bound, determined and caused by history. They forget C (the reaction of the Right* at the moment and the dynamics it may cause in the electorate) as well as D (the mutable dynamics of a fickle American electorate, etc.)
Seriously though, many Democrats seem to believe that more people getting frustrated with and tired of the war will inevitably cause electoral victory for them. I could be wrong and it really could be that simple but it seems to me that their notion of cause and effect will not take place in a vacuum.
*Taking reactionary positions can easily suit the Right.