The Senator who comes to town is "Mr. Benton, an actual United States senator". Thomas Hart Benton is indeed an historical figure, the first United States Senator from the state of Missouri.
Senator Benton is briefly mentioned in Chapter 22, which recounts the boredom of a long summer for Tom. Abandoned by Becky Thatcher, who is spending the season at her Constantinople home with her parents, and tormented by "the dreadful secret of the murder" he has witnessed, Tom is in a miserable funk. He looks for diversion in the normal events of town life, but nothing keeps his interest for very long. A "Negro minstrel show" comes to town and (makes) a sensation", but the thrill of it wears off after only two days. Even the "Glorious Fourth" lacks luster, for it is raining, and the arrival of "the greatest man in the world - as Tom suppose(s) -Mr. Benton, an actual United States senator, prove(s) an overwhelming disappointment - for he (is) not twenty-five feet high, nor even anywhere in the neighborhood of it". Tom has built up the prestige of the famous statesman to such an extent in his mind that the reality of the man cannot help but be a letdown. The arrival of a traveling "phrenologist and a mesmerizer" are no more stimulating, and the final ignominy comes for Tom when he catches the measles (Chapter 22).