The importance of the Senate has been demonstrated well, these last 4 years. They serve as the primary check on the Executive branch and, thus, the primary guarantor of good government.
270towin.com has two maps available:
In essence, these maps are saying that the people of Colorado are strongly blue right now, but history tells us that they’ll be pushed way back to the right over the next few months, via advertising and political pressure; Montana is already friendly to the left but will become more friendly to the right; Arizona is leaning left and will be a hard push to move rightwards but should go into balance and be a toss-up.
I would, personally, advise that you ignore the Presidential race and contribute money to the Senate races of Arizona and Colorado, first, and Montana if you feel like getting rid of money. Which side to donate to, I leave to your preference, but those would seem to be the battleground states for the Senate.