Yes, even in the 18th century there was vitriolic hate mail sent to politicians. Here's an example of one sent to Pitt around the time of the Corn Bill (excuse the terrible grammar and complete lack of punctuation, but I try to keep transcriptions as faithful to the original as possible):
“I say Billy, you infernal Blackguardly Son of a Thief you Bottled Nose Lanthorn [?] Jaws pimping looking Rascal Prime Minister egad my Old Buck you’ll be getting along side of the Devil soon but he’ll shave your Timbers for you and give you s Broadside and send you - but he will not send you to Windsor to drink 3 bottles of Wine after Dinner 3 Bottles of Wine for such a Damned old Punt as your while many a Poor thing cannot get Bread and all through you you infernal Imp bringing encouraging and passing the Corn Bill and thereby forming an Act of Parliament thereof encouraging those Rascals who ought to support inside of trampling on the Poor and all because you suppose if there is any Riot you can call upon the Volunteers to aid and assist you but know [sic] you meagre looking hound you are of the wrong side of the hedge for they will assist them and will be no longer Domineered over by such a Babboon as you if the Corn Bill is not suppressed the next meeting of Parliament you will hear of something you little think I render Rag Tag and Bob Tail Billy you infer on our Packet but your superior and Honest you Blackguard Damn [oe] God Bless King George Huzza.” 
1. William Dacre Adams Papers. Unknown sender to William Pitt (undated). The British Library Add Ms 89036/1/17, f. 56.