No. 4 Richard Lugar

If Obama were to appoint a Republican to the Secretary of State post, Indiana Senator Richard Lugar would be the obvious choice. Not only is he the GOP's ranking member - and former chairman - of the Senate Foreign Relations Committee, Lugar has had a warm personal relationship with Obama since he first became a senator in 2004.

It was Lugar who invited Obama to the Foreign Relations Committee and took the young Illinois senator to Russia and other former Soviet republics to inspect nuclear facilities. Lugar has praised Obama's foreign-policy approach and Obama in turn as trumpeted his relationship with Lugar as evidence of his bipartisan credentials.

But appointing a prominent member of the opposition party to be Secretary of State, the top cabinet post in any administration, would be rare if not unprecedented since the early days of the republic. In addition, Lugar has indicated that he would like to serve out his term and then seek re-election in 2012.

This Obama campaign ad highlights his relationship with Lugar, who has long made nuclear nonproliferation his legislative priority during his 32 years in the Senate.