Presidential Vs. Senatorial Races
Though Democrats will probably pick up more House seats than they will lose, Republicans will still probably retain a majority in the House. The story is roughly similar in the Senate, but with the parties in reverse roles: Though Republicans may perhaps (though that is less certain than the likely Democratic gains in the House) make a net gain of one or two seats, but the Democrats will still most likely keep the majority. Currently, realclearpolitics lists based on polls, 47 Senate seats as likely Democratic, 43 as likely Republicans and 10 as "toss ups" (the 47 and 43 numbers includes seats not up for election this year). Assuming half the "toss ups" are won by Republicans and half by Democrats, this would imply a Democratic majority of 52-48, compared to the current 53-47 majority.
This means that regardless of whether Obama or Romney is elected President, their power to change particularly domestic policy will likely be constrained by one of the chambers of Congress being held by the other party.
Usually, people vote for the same party in all elections, which is why the solidly Republican state Texas is near certain not only to be won by Mitt Romney in the Presidential election, but also to elect Republican Ted Cruz for the U.S. Senate. and why similarly the solidly Democratic states of California and New York are not only near certain to be won by Barack Obama in the Presidential election, but also by Democratic candidates Diane Feinstein and Kirsten Gillibrad respectively in the Senate election.
However, sometimes Senate candidates from one party do better than their Presidential nominee. Massachussets and Connectitut are both basically left-liberal states and Obama therefore currently has an average lead of 15 and 12 percentage points, respectively. Yet though Democrats Elisabeth Warren and Chris Murphy both have a lead in the polls against their Republican opponents Scott Brown and Linda McMahon, that lead is far slimmer than Obama's lead, only 2 and4 percentage points respectively. Meaning that though Romney has virtually no chance of winning over Obama in those states, McMahon and Brown has a shot of winning their Senate elections.
I'm not entirely sure why Chris Murphy and Elisabeth Warren are doing so much worse than Obama, but in part it likely reflect that their campaigns haven't been well run, and particularly in the case of Warren it also reflects that her opponent Scott Brown has tried to present himself as more moderate than the rest of his party.
By contrast, though Romney has a significant 10 percentage point lead in Missouri, the Republican Senate candidate Todd Akin trails his Democratic opponent Claire McCaskill by 5 percentage points. The main cause of why Akin is doing so much worse than Romney is of course his infamous gaffe about rape and impregnation.