Why did Moshe assume that Pharaoh would not listen to him? What Kal va'Chomer did he make?
Rashi (according to the Sifsei Chachamim): If Yisrael, who were being given good news, did not listen to him, how much more so Pharaoh, for whom the news was bad!
Ramban #1: Since Hashem had not engineered that Yisrael should listen to him, it went without saying that Pharaoh would not listen to him either, let alone the fact that he had a speech defect and it was not befitting for him to speak before such a great king.
Ramban #2: He thought that Yisrael did not listen to him on account of his speech defect, despite the fact that he gave them comforting news, and if he could not convince them, how would he convince Pharaoh, for whom the news was not so good.
Seforno: He thought that Yisrael did not listen because he failed in his first Shelichus. In which case, Pharaoh, who got his way, would certainly not listen to him.
Alshich: Bnei Yisrael are believers Bnei believers, and they know that I am the redeemer, for I said Pakod Pakadti, and they did not heed me - all the more so Pharaoh, a denier, will not heed me!
Gur Aryeh: There was only one reason for Bnei Yisrael not to heed him (Kotzer Ru'ach). There were two reasons for Pharaoh not to heed him - it harms Pharaoh, and due to Moshe's speech impediment.
Moshav Zekenim: Bnei Yisrael could not believe that Hashem would pick a Shali'ach who cannot speak properly. All the more so Pharaoh would not believe this. 1
That is, only eloquent speakers talked with Pharaoh, or he would be insulted by such a Shali'ach. (PF)
Hashem already informed Moshe that Aharon would speak on his behalf, so why did he again mention his speech defect?
Ramban: The first time Hashem instructed him to speak to the people, Moshe complained that he was not a man of words (See Shemos, 4:10), to which Hashem responded by appointing Aharon as his interpreter (Ibid. 4:16). However this time, Hashem ordered him to speak personally to the people (above, Pasuk 6), but without success. Consequently, when Hashem now instructed him (in Pasuk 10) to speak to Pharaoh, he complained once more about his problem. This time too, Hashem responded (in the next Pasuk) by sending Aharon together with him to speak on his behalf.
Seforno: Because he thought that Aharon only spoke on his behalf on the previous occasion when he spoke to the B'nei Yisrael, but that when he spoke to Pharaoh, he would have to do so himself.
Rashi writes that Moshe made a Kal v'Chomer that Pharaoh would not heed him. There is a refutation - Bnei Yisrael did not listen due to shortness of breath and hard work!
Mizrachi: Moshe did not know that this is why Bnei Yisrael did not heed him.
Alshich: Moshe realized that perhaps there is a refutation, so he gave another reason - I do not speak properly.
Gur Aryeh (1st answer): Any stringency refutes a Kal v'Chomer, even if there are greater stringencies on the other side, for we do not know the reasons for Torah laws. This (whether or not they will heed) is not a Torah law, so a big stringency (Moshe's words benefit Yisrael) outweighs a small one (Kotzer Ru'ach); the Kal v'Chomer is sustained.
Gur Aryeh (2): Due to Kotzer Ru'ach, a person detests what he hears. Pharaoh had two reasons to detest what he hears, each no less than Kotzer Ru'ach. (Refer to 6:12:1:6 and the note.)
Da'as Zekenim, Hadar Zekenim: Kotzer Ru'ach was due to the hard work - all the more so they should have accepted his words that they will be freed!