This is how it would work.The Electoral College meet on Dec. 17. The votes are cast, and it is generally accepted that each state will honour the voters will, i.e. cast the vote for the candidate that won the state e.g. if Obama wins the popular vote of California, he gets all of California's Electoral college vote (55). However only 26 states have laws requiring the electors to vote for the person they were chosen to support. The other delegates can go rogue and vote for someone else and that would be a real conundrum ( the Associated Press has already found 5 potential rogues) But let's assume that the vote is cast in the way it should. If neither candidate has 270 votes, the 12th amendment requires the House of Representatives to convene to elect the President. He must receive at least 26 votes ( as mentioned above, each state has one vote)
The Senate then chooses a vice-president. So you have a Republican Romney as President and a Democrat Biden as vice-president. Go figure and let the dance macabre begin.