At the moment the president election campaign in the US is in the full swing. As a foreign national I am a bit surprised that current US president (Obama) seem to actively participate in agitation. Just as a side note in Russia such behavior is prohibited by law since it is assumed that current president could have too big influence over the people's minds thus it would give too big advantage to specific candidate which he or she supports.
So, can anyone clarify for me please if US have some laws which cover participation of current president (or some other higher-level government member) in the election campaign and agitation?