Behold now I have taken upon myself to speak to the L-rd, although I am but dust and ashes. (18:27) All people are but dust and ashes, yet they speak to G-d daily when they pray. Abraham was pointing out that, despite the wickedness and corruption of the people of Sodom, Abraham felt enough compassion for them to pray on their behalf.
Abraham was saying that if he could feel compassion for those people, then so should G-d, Ruler of the universe, grant them mercy and allow them to live. (Reb Bunim of P'shischa)