Barack Obama, King of America

Ra'avad

The principle is that anyone who is important in his/her ruling power like a king, and has the power to send someone to be killed while nobody has the direct power to overturn his orders is considered to be like a king. You should say a blessing when you see this person (in person) and should give him/her respect

Rav Moshe Shternbuch, Teshuvos VeHanhagos 2:139

I remember when the President of the United States came to America for a visit, and the rabbis disagreed on whether to treat him like a king who we must give high levels of respect to. My position is that, since the President of the U.S is only in power for a limited period of time, and is based on a decision of the people, and because his powers are limited constitutionally, we do not need to say a blessing when we see him. He is not parallel to the English king/queen, even in our time, as any new piece of legislature must be signed by the king/queen, and the power is passed down hereditarily. Furthermore, the queen is actually royalty, unlike in America where it is just an elected office...thus the American president is not at all like the British queen, and is not considered royalty at all

Rav Ovadia Yosef, Yechaveh Daat

It appears that, since the U.S President has the ability to grant a pardon to a person who has been sentenced to death by the courts, it is apparent that the power of life and death is put into his hands, and he is thus like a king who has power over life and death according to the law. Even though the president is chosen by the people for a proscribed period of time, and he/she cannot stay forever like a king/queen, while she is the president a person should make a blessing on her that they would make on seeing a queen

Shu"t Divrei Chaim, Yoreh Deah 1:30

When the rabbis wrote in their legal decisions that it is forbidden to follow the ways of the nations, their intention was not, heaven forbid, to not give their leaders respect. Conversely, the rabbis warned us many times to respect the leaders and citizens of the nation, even included the nations that have made our lives much more difficult and painful. Even in spite of all of the terrible things that the Egyptians did to us, when God took us out and ensured that we were no longer afraid of them, God nonetheless warned us not to hate the Egyptians and to continue to show them respect since we derived benefit from their land. If this is the rule regarding the Egyptians, how much more so should it apply to the nations that have made our lives easier, and who respect our God, who have spread out their wings to protect us from those who wish to harm us, these nations which love kindness and respect our Torah, may God grant them their appropriate reward. Such nations we should certainly respect them and praise them, and thus clearly the rabbis would want us to show proper respect to such nations, and they certainly would not want us to refrain from joining civic organizations. It clearly must be the opposite! We know from our tradition that the greatest sages of the Talmud, like Rava and Shmuel, like R' Abahu and Rabbi Yehoshua ben Kisar were part of the government, and thus there is no problem in welcoming nations to join forces with the government.