You may intend it as a vote "against" Obama, but it will most assurely be a vote FOR Romney.
Sad to say that voting "against" is a popular sophistry. There is no voting "against" in elections; you can only vote FOR something to replace what you want to vote "against." You don't even vote "against" a ballot proposition -- you can only vote FOR nixing it. It's a fine point, but a crucial one in understanding the price of ABO.
It is a hard reality that a lot of ABOers may wake up to with a nasty nightmare jolt come 2014 when it's not "about" Obama anymore, but only about Romney and how conservatives are the minority in an administration where moderate Republicans, the White House, and Democrats "unite" to advance a "progressive" agenda.
I'm voting the only way that makes sense in an O v R contest: third party in order to deny a popular mandate to whichever guy wins. The only leg-up conservatives in Congress can have for fighting the next next president, who is guaranteed to want to advance government tyranny, will be having the numbers to back them up in moving against a president who lacks a popular mandate, a president whom the majority of Americans rejected at the ballot box.
If Obama wins, the regime, together with their supporters in the media will hype it as a mandate, even if its a razor thin margin in both the general election and the EC, so your rationalizing your third party protest vote as denying either side a mandate doesn’t matter.