Exodus 6:12

And Mosheh said before the Lord, Behold, the sons of Israel do not hearken to me; how then will Pharoh hearken to me, and I a man difficult of speech?

BEHOLD, THE CHILDREN OF ISRAEL HAVE NOT HEARKENED UNTO ME because You have done nothing [for Israel] so that my words should be acceptable to them. AND HOW THEN SHALL PHARAOH HEAR ME? AND besides, I AM OF UNCIRCUMCISED LIPS, and I am not fit to speak before a great king.
It is possible to explain that Moses thought that due to his deficiency — for he was of uncircumcised lips — the children of Israel would not hearken to him, for he could not frame [words] to speak kindly to them, words of good cheer and comfort, and all the more how could he speak to Pharaoh?
Now the reason that Moses again broached this argument [of his speech-impediment, when he had already mentioned it above in Chapter 4, Verse 10], is that G-d did not originally command him that he was to speak before Pharaoh. He merely said, And thou shalt come, thou and the elders of Israel, unto the king of Egypt, and ye shall say unto him. In that case it was possible that the elders should speak [before Pharaoh] and Moses would remain silent. Then Moses said that he was ashamed even to speak to the people, saying, I am not a man of words., 4:10. Whereupon the Holy One, blessed be He, answered him, And he [Aaron] shall be thy spokesman unto the people., Verse 16. And so indeed Moses and Aaron did when they first came to the people, as it is said, And Aaron spoke all the words which the Eternal had spoken unto Moses, and did the signs in the sight of the people., Verse 30. But now Moses himself was commanded, Wherefore say unto the children of Israel: I am the Eternal, and he did speak to them as He commanded him, but they did not hearken to him. Now when He again commanded Moses to speak to Pharaoh, Moses said, “How can I speak to him, I who am of uncircumcised lips.” Then the Holy One, blessed be He, associated Aaron with him, and gave both of them a charge unto the children of Israel, that they should say to them whatever He will command them, and unto Pharaoh to bring them forth out of his land. Now Rashi commented: “[And He gave them a charge unto the children of Israel, and unto Pharaoh, king of Egypt]. That is to say, He gave them a command with regard to the children of Israel and His mission on which He had sent them to Pharaoh.” But there is no need for this.

הן בני ישראל לא שמעו אלי, he believed that this was due to the fact that from the moment he had started speaking to Pharaoh the latter had become worse in his attitude and the people, putting two and two together, had concluded that his mission had failed. If he felt that way, Pharaoh would certainly not pay any heed to him and continue doing what he liked.

This was especially so, seeing that he was also an ערל שפתים, afflicted with a speech handicap. He mentioned this as he did not believe that Aaron would accompany him other than for the time he identified himself as G’d’s emissary when he spoke to the people.

הן בני ישראל לא שמעו אלי, “seeing that(even) the Children of Israel do not listen to me, etc.” According to the plain meaning of the text what Moses is saying is that the people do not listen to him, so what chance is there that Pharaoh would listen to him. In the event that one would excuse their failure to listen to him to their state of mind, i.e. they did not have a chance to indulge in day dreams, he adds that he attributes their failure to listen to him to his speech impediment, and therefore does not blame them. Alternately, perhaps Moses was not aware of the Torah having told us, the reader, that the reason why the people did not listen to him was their state of mind, i.e. impatience. I have heard from my teacher a Rabbi from Lisbon, that what Rashi meant when describing Moses’ logic here as unassailable, that the reason given as their state of mind, was an additional reason. His logic went as follows. Normally, when a potential redeemer promises his people that he has come to redeem them from slavery, they will be overjoyed. Seeing that the Israelites’ reaction to his message had been so negative, how much more negative would Pharaoh’s response be to their demanding a holiday!