It's unlikely that the President carries. USSS protection is awfully good, and unless TSRHTF the president's first duty is to preserve himself for the good of the country. He should be getting out of the way while agents protectively pile on top of/around him. There's also a chance that the Prez may be wired up with something or wearing some sort of exotic armor which a large metal firearm may interfere with.
With senators and lower-level people, it's a bit different. They just don't have the same type of heavy protection, and could potentially find themselves in a situation without a bodyguard nearby. There's also a better-than-average chance they could be targeted for assassination by someone who is particularly POed at them, or just nuts.
I'd carry, if for no other reason than to make a point of supporting RKBA. I'd also build an armory in the WH to store my stash. The Secret Service would be jealous.
Of course, if I had the resources to become president, I'm fairly sure I'd also have the $$ to own at least part of a manufacturer of quality firearms, and definitely an FFL retailer. One day... :happy: