Throughout his first term, Obama will be constantly pushing truly massive new spending projects. More banking bailouts, the auto bailout, bailouts for other industries, socialized medicine, pork ridden budgets, global warming funds -- it goes on and on. If Republicans oppose those very same massive spending projects, they can make that the core of their campaign in 2010.
Now, some people would say that won't work for three reasons:
1) The American people don't care about pork projects. It is true that few representatives are ever going to lose a state or a district because they voted for too much spending.
But, and this a big but -- what's good for a particular politician on the micro level can be extremely damaging for a political party on a macro level if the behavior spreads.
Put another way, people don't mind when THEIR representative brings home a "Bridge to Nowhere," but boy do they hate the idea of OTHER PEOPLE benefiting from government largesse.
Moreover, with the Democrats in charge, these big spending bills will inevitably be incredibly wasteful and full of corrupt "one hand washes the other" spending. That sort of obscene corruption and grotesque back scratching certainly isn't going to be what most Americans thought Barack meant when he said he was going to "change the way Washington works" and they will be offended by it.
2) The Republicans have no credibility on deficit spending after the Bush years. The GOP's reputation in this area certainly took a huge, much deserved hit during the Bush years.
However, A) Obama is going to be the biggest spending President in American history by far and B) People are going to pay a lot more attention to what happens in 2009 and 2010 than what happened between 2001-2008. If the GOP continues to oppose these wasteful bills as the Democrats continue to support them, the American people will give credit where credit is due for fiscal conservatism. Do keep in mind though, even if the GOP turns into a party-full of Jim DeMints and John Shadeggs, it will take several months for it to fully sink in with the American public after the poor performance of the Bush years.
3) If the American people really cared about spending, they wouldn't have sided so heavily with the Dems in 2008. The Republicans talked a lot about cutting spending and small government issues throughout the Bush years, but they didn't walk the walk. You can't credibly run as a small government, deficit-cutting party when the government expands under your watch and spending explodes.