Democrats kept the White House. They picked up 2 seats in the Senate and around 5 to 7 seats in the House. Also, they did quite while in Statehouses across the country. Does that mean there was a small Democratic wave (call it a splash) that swept the country? Did legislators ride the President's coattails to victory?
Best argument for - Obama's turnout machine helped them win down-ticket races.
Best argument against - throughout the year, poll in each race moved independently from the other races.
It's a matter of opinion, but I am going with a "No" on this one. I believe that all of these wins were independent events, like the polls showed, that happened to coincide. Here is why:
1) The Presidential Race - Romney was not that strong of a candidate. People still blame the last administration for the financial crisis. The economy is slowly growing. There is no major foreign policy problem. All predictions pointed to a small victory for the incumbent with these factors in mind.
2) The Senate - The Democrats picked up seats because Senate candidates in Indiana and Missouri, who were both favored to win, punted their campaigns when they made public comments about rape. Without those two seats, the Senate ends up being a wash. Jon Tester (MT) and Heidi Heitkamp (ND) both won in very conservative states, which lack the regular Democratic consistencies. There were no Obama GOTV efforts for them. These candidates held their own. Candidates matter. (Note Angus King in Maine, too.)
3) The House and Statehouses- There was a huge Republican (or tea party) wave in 2010. Republicans made headway in traditionally Democratic territory. Everyone knew that it was never going to last. The Republican electorate advantage from that year receded from Republican +7 that year to being about Dead Even this year. Because of this, some analysts originally expected a counter-wave due to the size of the swing alone. But Republicans controlled post-census redistricting in many of the State houses, so they were able to mitigate Democratic gains through gerrymandering. The Democrats were still able to pick up a handful of seats.
The President had no coat tails.