This week, Democratic Presidential candidate Barack Obama has been engaged in a European and Middle Eastern tour, where, among other activities, he has been meeting with foreign heads of state. This is completely inappropriate, seeing as he is not currently a head of state. He has already met with at least Angela Merkel (Germany) and Hamid Karzai (Afghanistan). He is a senator, not the President (no matter how much he might think that he already is).
On the world stage, only official heads of state ought to engage in diplomacy with one another. Aspirants need to wait until they are elected in order to do so. To do so lacks accountability and can breed confusion as some could think that his statements reflect accurate and current American policy positions.
This problem was seen in Canada during last year's climate conference in Bali. Stephane Dion, even though he was not a head of state arrogantly attended the conference feeling that he had the right to speak on Canada's behalf. Having two different delegations to represent Canada, one official and legitimate and the other intruding and out of place, did not make Canada look strong and respectable on the world stage.
Domestic politics are for within the country. Politicians should not attempt to engage in foreign policy without a mandate to do.