I want to write this now, before we are swamped by the usual punditry. Whoever wins on November 2, the only incontrovertible lesson is that the winners got more voters to the polls than the losers. Anything else is just speculation.
The opinion polls indicate major Republican gains, which should be expected for several reasons. With few exceptions [1934. 1998, 2002] the President's party usually loses strength in Congress in mid-term elections. When the economy is bad, more incumbents lose. When the public feels the country is "on the wrong track" -- and that figure is now above 60% -- the President's party suffers.
What the national polls can't show, however, is why Candidate A beat Candidate B, since that outcome depends on multiple factors -- some local, some national, some related to personalities, some to ideologies.
So beware the instant analyses that try to draw broader lessons. Most will be the prescriptions the analyst would offer before the voting ["be more centrist," "be more conservative," "be more liberal," "be more negative," "be more positive"]. Instead, just congratulate the winners you like and console the losers you wish had won.