Interesting analysis here from Sean Trende, especially at the end:
If the president’s job approval is still around 43 percent in November — lower than it was on Election Day in 2010 — the question would probably not be whether the Democrats will hold the Senate, but whether Republicans can win 54 or 55 seats. Given the numbers right now, that should not be unthinkable.
But there’s a flip side to this. If Obama’s job approval does bounce back — which is exactly what happened in 2012 — there’s a reasonable chance that Republicans could walk away from this cycle with just a handful of pickups.