You might be surprised; there is a long tradition of mudslinging in politics. Notably there was the Republican attack of Grover Cleveland in 1884 when it came to light that he had an affair, and the child had gone to a mental institution. He was hounded on the campaign trail with chants of "Ma, ma where's my pa?" This is almost certainly beneath the level of discourse used even today. These chants were led by Democratic Party supporters and not Samuel Blaine himself, maybe because he was in the midst of a cash for political favors scandal himself.

If that is later than you were looking for, then we can go back to the election of 1828, which was among the dirtiest campaigns in American history. Andrew Jackson was running against the incumbent John Quincy Adams, and the campaign was full of mudslinging. JQA came out with the Coffin Handbills(many of which are available on the Cornell University Library website) which attacked Jackson for his execution of deserters, his massacre of Indians, and his marriage to his wife, Rachel--who's divorce was not finalized at the time of marriage. Jackson hit JQA back, attesting, among other things, that in his time in Russia JQA had given an American girl to the Czar as a mistress. This was truly an awful election niceties wise. If you are interested in how it pertains to our modern systems of campaigning, I highly recommend Lynn Hudson Parsons' book The Birth of Modern Politics