In 1888, the British ambassador to the US received a letter from someone claiming to be an English immigrant living in California. It was asked in the letter if it would serve British interests if Grover Cleveland were to be re-elected. He sent a reply suggesting that it would indeed be better for England if the Democratic incumbent won the election.
The letter turned out to be from a Republican operative. The ambassador's reply was made public, and it caused Irish-American voters to oppose Cleveland.
The moral of the story is that it can backfire when government officials try to influence the internal affairs of another country. President Obama is right to condemn the violence taking place in Iran, but that is all he can do about it at this point. With an embargo already in place, it would be unwise to make any threats to the Iranian government that cannot be carried out.