After taking such a beating in the last election, and being left with a homogenous "conservative" party, the Republicans had to find a way to oppose Obama or else they would have doomed themselves to at least eight years of failure. It might happen anyway, but they at least have a chance now.
Think about it.
If the Stimulus plan works, Obama and the Democrats who are in the majority will get credit for it, and for the Republicans to be included only validates the status quo. What would their argument against the Democrats be if both sides worked together on the biggest political issue of the day, and it worked?
If the Stimulus plan fails but the Republicans were equal partners with the Democrats then they would have forfeited their right to criticize and assign responsibility, because they would have ownership.
So in both success and failure, the Republicans come out on the short side in terms of politics if they actually worked with Obama. Bipartisanship is in Obama and the Democrats' favor because the default setting is Democratic Rule.
Therefore it only made sense to figure out a way to unite against the plan.
This way, if it succeeds you can argue (probably credibly) that the government had little to do with the economy rebounding. Afterwards you change the topic to some shortcoming.
If the economy is still going poorly in 2 years, then Republicans are in position to offer change, and blame Democrats for not trying their ideas.
With this being the case, Obama should continue to look as though he is offering bipartisanship, but all along doing what he thinks is correct and will work, because one way or another Democrats will be held accountable and Republicans aren't dumb enough to choose a lose-lose situation.
I don't think.