1) Beginning in the 1820s, Martin Van Buren and other pols who were afraid that the declining political power of the slave states was going to result in either a) the north abolishing slavery, or b) the south secedeing because of slavery, set out to create a national political party that would elevate personal greed/job security over ideology (views on slavery). That party was called the Democrats. To "get out the vote," the Dems relied on a number of tactics, but for our purposes the most important was the creation of a "partisan press," whose sole job it was to get Dems elected. They ONLY covered Dem politicians (favorably). After a while, a rival press, run by the Whigs, started to do the same for the Whigs. Either way, neither press reported "news" but rather PARTISAN viewpoints. That was their reason for existing.
2) In the Civil War, the demand for real information and facts, coupled with the widespread use of the telegraph---which required an economy of words ("just the facts, ma'am")---led to a new journalism based mostly on facts, with "editorial" now pushed to the last page. Over a 20 year period, most papers in the country (which had previously been subsidized by the parties) started to increase circulation and drop their subsidies. To get bigger circulations, you had to be "fair and balanced" and not offend a large sement of your readers who might be either Republicans or Democrats. By 1910, journalists had adopted codes of ethics, reporting guidelines that insisted on MULTIPLE known sources, "getting both sides of the story," and basic fairness. Papers REPORTED, they didn't PREACH. (Again, you have a couple of exceptions, such as the Hearst papers). 3) This lasted until the 1960s, when for reasons not entirely known (or which I haven't yet proven) the journalists began a rapid shift to the left. I think this happened early, under JFK, NOT later, during Vietnam and Watergate. But either way, by 1975, the major media was decidedly leftist.
In the 1980s we started to enter a fourth era, not completed yet, in which the "new (conservative) media" has fought back and is now offering alternative interpretations of news. We are, in many ways, back to the 1830s, except that people today THINK that "news" is, well, news and not propaganda.