Is the smearing growing in the USA (in the last decades/centuries)? Or at least the perception about smear campaigns?
As far as I know, nobody has yet developed an impartial metric for the negativity and untruthfulness of campaign statements. Indeed it's difficult to see how anyone could do it objectively. However there does exist evidence that the US electorate is generally getting more partisan, both in terms of districts, and in terms of how liberal or conservative the supporters (and politicians) of both major parties are.
There is also evidence that prior political beliefs affect how people interpret the favourability and veracity of news stories. It might be putting two and two together and coming up with five, but this would at least support an increased perception of smear campaigning (people feel more strongly about candidates, so negative stories are viewed as smears by the opposition rather than simple reporting).
Not really. Political campaigns used to be much nastier than they are today. The primary difference seems to be that with modern technology it is possible to hear such things direct from the candidates. A hundred years ago, you would have read it in a newspaper and the newspaper would more likely be the source than the candidate.
Try doing an internet search on "history of political slander". Google has quite a few results.
That gives examples from the elections of the two Presidents Adams. Both engaged in quite a bit of mud-slinging with their opponents--and that was 1824 or earlier.
If anything, modern elections are much more polite. The modern media is less explicitly partisan (although they may be biased). In 1860, people knew which newspapers were Republican and which were Democrat. They achieved balance by usually having at least one of each. Fox News and MSNBC were the rule back then, not the exception.