Mitt Romney is desperately trying to make the election a referendum on the economy. Whenever possible, he is refusing to give any details about his plans, choosing instead to focus on poor current conditions. He and his running mate today both refused to answer direct questions about which tax deductions they would eliminate to pay for their promised income tax rate cuts. They can (maybe) get away with this because, thanks to our voting system, there are only two options perceived as viable, so all one side has to do is damage the other side so badly that they win by default. Republicans have dedicated the last 4 years to this purpose.
If we were to switch to a system which encourages multiple parties, this destructive, all-negative strategy would be impossible. Devoting one's resources to tearing down one opponent doesn't work when there are more than one alternative available. In a theoretical race between Romney, Obama and Candidate X, Romney could spend all the time he likes tearing down Obama, but he would then have to be specific about why he's better than Candidate X. With even more parties, negative campaigning becomes less and less effective and presenting one's own vision becomes the only real option.