Say you wouldn't take the job and why. Everyone has something (generally narcissistic) that they'd like to do if they were president. Humble out and turn it down, there are a gazillion reasons not to as well.

It doesn't show a lack of ambition, not everyone wants to be president, I sure as hell wouldn't, but it's not because I lack ambition but more so because I have integrity and credibility that would be jeopardized in such a role.