Putting up Santorum, for example, on an "anyone but Obama" platform might be a losing strategy, but there's a key difference between "anyone but Obama" now and "anyone but Bush" in 2004:

In 2004, the economy was growing and all discontent with the President was over a heavy-handed foreign policy that about half the country supported anyway. The discontent was removed from the voters' wallets. In 2012, Obama has saddled a massively expensive new Federal program on voters, the economy is recovering but the jobs outlook is still anemic, Obama just talks about spending more and more money without even admitting that entitlement reform is needed, and many people are still nervous because so many legal changes have been threatened (should I buy a house if the mortgage deduction goes away? How much will it cost me to hire an employee due to Obamacare? Will the capital gains rate go up?).

It may sound cliché to say that people vote with their wallets, but it is no less true for that. If the economy is good, people can vote for all sorts of other reasons (I like his haircut, he doesn't want to murder babies and neither do I, he supports gay marriage and so do I), but when a voter is nervous about the economy, it trumps everything else.