I will say it right now. If Obama is reelected, Democrats will hold the Senate, losing at most two seats(ND and NE). The reason is that if Obama is strong enough to win, he will be competitive in states like Montana, Missouri, and will likely win Virginia and Florida, pushing Democratic incumbents over the finish line.
If Democrats were to lose the Senate while Obama was being reelected, it would be the first time since the direct election of Senators that a President got reelected and his party lost control of the Senate.
I dont why people assume that Democrats are destined to lose the Senate even if Obama wins reelection. In 2008, Obama won every state that Democrats have an open Senate seat in with the exception of North Dakota. He also won every state where there is a Democratic incumbent up for reelection except for Nebraska and West Virginia(where Machin is super popular).
I dont get all of this handwringing. In fact, my current prediction is that Democrats lose just one seat in the Senate(NE and ND) while picking up Arizona, which Obama has a good shot of carrying. Hell, Democrats could even gain a seat if they pick up Nevada and upset Scott Brown in Massachussetts.
So you heard it here first. Democrats will lose at most two seats in the Senate.