The chart above shows the poll averages (from RealClearPolitics) in 12 states with a Republican senator up for re-election this year. The Democrats need to take five of these seats to once again control the U.S. Senate.
Currently, Democrats have a significant lead in two of those races -- Wisconsin and Illinois. They are close in four other states, with some polls showing them leading and some showing them behind -- Missouri, New Hampshire, North Carolina, and Nevada. The other six races seem to be favoring Republicans rather significantly right now --Ohio, Florida, Iowa, Georgia, Kansas, and Arkansas.
If Hillary Clinton is elected, and I think she will be, she is going to need the Senate to be controlled by Democrats. Without that, she won't be able to do much. The Republicans will just do four years of obstructing anything she tries to do (just like they have done for the last few years to President Obama).
It seems obvious to me that the Democrats need to put more emphasis on many of these Senate seats.