Both sides in the presidential contest are eager to declare this election the most momentous event since the discovery of fire, and we won’t know how true that is until we see how badly we’ve been burned. But I think it’s already apparent that the campaign will be considered a milestone in the history of the U.S. media.

Here’s what has changed:

The mainstream media no longer play a key role in setting the national news agenda. The established news media were nowhere on public-policy matters. Issues that should have been their meat and potatoes – such as the adequacy of homeland security or remedies to stanch job losses – were largely untouched. A recent BBC Online critique was titled, perceptively, “How the U.S. media lost the plot.”

Instead, the agenda was set by partisans, via political advertising and committed freelance efforts. Time and again, established media essentially reacted to issues rammed through by outside groups. It started with the Howard Dean primary campaign, in which a grass-roots protest against the war blossomed briefly into an electoral insurgency.

Fahrenheit 9/11 threw the fat in the fire, raising President Bush’s character and competence as reelection issues. Also, that critique, like the swift boat group’s anti-Kerry assaults, leapt over the media firewalls and forged the campaign debate.

One lamentable conclusion: Buying your way onto the national agenda is easy; it just takes money.

The horse race defeated all comers. I can’t exaggerate the degree to which mainstream news evaluated virtually all candidate actions, utterances, proposals and disclosures by how they might affect not the country, but the vote.

Learned commentators speculated endlessly on the impact of a Kerry health-care proposal on Ohio’s turnout or Pennsylvania’s undecideds, without ever looking at the proposal itself.

Political writer Matt Taibbi has skewered the almost derisive way in which journalists referred to policy “details,” which they never actually described (and probably couldn’t).

News media credibility was in freefall. Unrelenting attacks on so-called liberal bias were partly responsible.

But the huge irony is that for all the gnashing of teeth over the “60 Minutes” anti-Bush National Guard memos, the two most calamitous errors that the media have made in this young century were both immensely favorable to Bush: prematurely awarding him the 2000 election and shilling for his fallacious pre-war claims about Iraq’s strategic arsenal. When those various instances are set alongside the cases of reporter deceit and the still-unfolding newspaper circulation scandal, the media inspire anything but trust.

The Web stretched the universe of political news. The Internet has matured into a boisterous adolescence, with broad claims of diversity and public empowerment. We’ve entered what pioneer Matt Drudge once described as “an era vibrating with the din of small voices.” Bloggers are more adept at verification than original reporting, and on the Internet it’s not easy to know who’s pulling whose strings. But time and again the boundaries of coverage have expanded because of the persistence of Web-based reportage and commentary, which are now integral to any journalist’s beat coverage.

Partisanship is here to stay. Advocacy journalism may rankle, but its legitimacy within the national discourse seems to be more firmly established now than ever. Indeed, it’s the claim to impartiality that the public seems unwilling now to accept.

The supremacy of fact is under siege. People should argue over which facts matter, not what the facts are. When a University of Maryland study found that 72 percent of Bush supporters believe that Iraq had or was actively developing weapons of mass destruction, and 75 percent believe that Iraq was substantially supporting al-Qaeda – claims that not even the administration makes – something is wrong with the country’s political information system.

Hence, with the 2004 campaign, patterns of media influence – of who gets to speak and to be heard – fundamentally shifted, with once-authoritative voices discredited.

A robust new conversation conducted on the Internet, talk radio and cable TV has assumed historic prominence. Still, despite an unparalleled richness of information and multiplicity of perspectives, the discourse that characterized the current campaign season was dopey, squalid and mendacious.

And vast numbers of people believe important things that aren’t true.