Rav Aviner responded that the question actually applies equally tot he entire government, as the Prime Minister does not act alone. However, Rav Aviner said, while clearly one should not lie and e deceitful even to achieve such a position, once he has achieved the position he must be shown respect - after all, he is the Prime Minister. 

Rav Aviner compares this to the situation from Tanakh in which Absalom stole the hearts of the people and became king. The Yerushalmi says that at that time had David needed to bring a korban he would have brought an individual korban, not a kingly korban - even though Absalom won the office with deceit, he was in fact king.

So, again, while it is prohibited to lie and use deceit, once it has been done we must show him the honor and respect that the office demands. If we dont show him the respect due a national leader, our enemies will pounce on the opportunity of our disunity. 

source: Srugim