Like a lot of things, there were slow changes and then sharp breaks.

The best single volume history on this question is "See How They Ran" by Gil Troy. It documents the shift in election techniques from the resolutely inactive days of Washington to the frenetic two year hyper-campaigning of today. I'll summarize some of his findings and add a few other points.

it is true that the first five or six Presidents didn't "run" for office. They "stood" for office, putting their name out and staying resolutely silent. The echo of this approach remains in today's politics. Sitting Presidents don't campaign like challengers. They engage in a "Rose Garden campaign" of Presidential announcements and foreign tours at least until the nominating convention, leaving just two or three months for actual electioneering.

In the 19th century, campaigns were typically carried about by proxies who would headline rallies and make important speeches. Candidates might write letters that explained some of their policy positions but these were often broached as letters to friends that were then released to the general public.

Andrew Jackson was the first to really engage with the public during a campaign. When Stephen Douglas traveled for campaigning in 1860, he actually pretended to be visiting his mother. James Garfield traveled from Ohio to New York, and was the first candidate to make a series of "stumping" speeches successfully. William Jennings Bryan really took the "stumping" approach and the advances in the railroad and made it into the first campaign we might recognize. If there is a hard break from the traditions of the past, it is 1896 and Bryan.

What is interesting is that Bryan lost to a campaign that was based entirely in the traditional model of candidate passivity, with McKinley undertaking the "front porch campaign" where voters came to hear him speak from his front porch. But his campaign manager, Mark Hanna was a pioneer, using donations from wealthy business owners to develop a modern advertising campaign that defeated the insurgent Bryan.

It is important to emphasize that this decorum was politically motivated. The less a candidate said, the less trouble he could cause. By making no statements on policy, both supporters and opponents of a policy could be told the candidate agreed with them. There was no proof he didn't because no one could ask him and he made no speeches, promises or appearances. Troy argues that Clay lost to Polk because he wrote letters laying out his views instead of being silent.

We see the types of low key campaigns with little details in the 20th century from front runners. Thomas Dewey ran a very low risk campaign in 1948 on the advice of Republican insiders who felt the election was in the bag unless he made a major mistake. The result was Dewey looking distant and cold, and projecting a complacency that drained excitement and may have limited turnout.

As for lamenting the end of decency, yes. Every minor change in campaign technique was seen as a terrible decline in the moral stature of the Presidency and the country. But there never really was a time when candidates were actually above politics.

Even Jefferson and Adams were engaged in behind the scenes machinations and the level of rhetoric was deplorable. One newspaper said of a potential Jefferson victory: "Murder, robbery, rape, adultery, and incest will be openly taught and practiced, the air will be rent with the cries of the distressed, the soil will be soaked with blood, and the nation black with crimes." Adams was blasted as wanting to set up a monarchy and marry his son to George III's daughter.

Sources:

See How They Ran, Gil Troy