A physical lock is used to make something secure. Being "a lock" or having "a lock on" in American English is to be secure about an outcome. Considering all the turmoil during his presidency, Obama is certainly not a lock to win re-election in 2012.

lock = whore But why it is used here, I don't know.

Hi Jenisson. "A sure thing, a certainty". In the UK we would say: "Obama is not a 'certainty / dead cert' for the presidency." cert = certainty 'Cert', 'certainty' and 'sure thing' are also commonly used in the USA and are probably more common than 'lock'. We also say: "a dead cert" or "a dead certainty" again to mean that something is definitely going to happen. Here is the reference: http://books.google.co.uk/books?id=5F-YNZRv-VMC&pg=PA625&lpg=PA625&dq=lock+american+slang&source=bl&ots=mir-VCaIZo&sig=1ddXGvZ0os7hdGNF3gI3UzRbvcg&hl=en&ei=7PTMTf-eJsmJhQeEsOzyDA&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDoQ6AEwBQ#v=onepage&q=lock%20american%20slang&f=false

Dr. Bob has the right interpretation here. Have a lock is like a missile lock. The missile's radar is locked on its target.