You are the one who specified "landslide". I found that interesting and assumed that was a feature of the bet you had offered.
If the bet is merely over whether or not Romney wins, that's fine, but it'd be more interesting to if there's a range figure he reaches (positive or negative) for changes in odds. Or if there are charities/causes involved that we might despise (full disclosure, I'm libertarian, not liberal. Which would alter the likely composition of despised charities. I'm also not voting for either of them and would be doing this mostly to prove a point than because I actually care.)
Otherwise it's a pretty boring bet that you may as well just go on Intrade to place as you'd get around the demand that you put money in escrow for it.
I would put the odds for Romney winning at all at around 20-25%, and winning by more than a slim margin at almost zero. I think it is possible, not very likely that he wins but I'd be confident enough to bet against it. If the bet were interesting. If there's a Euro-related crisis before the election, his chances are better. If there's another foreign policy related crisis that actually gets attention between now and then (Israel bombs Iran?, Far East nationalism turns up as people shooting at each other), you can forget about it, Romney is totally screwed then.