The most important issues for Americans right now are the economy, the deficit and taxes. On all three the Republicans hold an advantage over Democrats according to Gallup.
This is the simplest visualization for why Democrats should be worried.
The economy has been the top concern for voters for years and it doesn’t look like that is going to change. The recovery under President Obama has been slow and many people have been left out of it completely.
I doubt that voters are really drawn to the Republican message on the economy but are more just disillusioned with Obama’s handling of it.