In case you haven't noticed, the stimulus bill really isn't that popular. According to a recent poll by Rasmussen, only 38% of Americans think it'll help the economy. A lot of that is pessimism in general, but you have to think that Republicans weren't just picking names out of a hat with this one.
Others have said this already, but Rasmussen is showing different results from everyone else on its polling. There are at least two and possibly three competing polls, one of them Gallup, showing the stimulus bill is enjoying great popularity. Perhaps people are concerned about whether it will work or not, but they prefer it to doing nothing, and rightly or wrongly that is how the Republicans are viewed.