Billions of dollars are spent every election cycle on advertising, which accounts for the lion's share of the money in U.S. politics. Campaigns also spend countless hours organizing voter contacts in hopes of winning over would-be supporters.

A new study suggests it's largely a waste — or something close to it.

The study from the University of California's Joshua L. Kalla and Stanford University's David E. Broockman challenges some of the most widespread preconceptions about political campaigns — starting, first and foremost, with the idea that trying to persuade voters is a good use of a campaign's time and resources. (A brief description of the study can be found here.)

The study's authors combined a hodgepodge of 40 existing experiments on the persuasive effects of advertising and campaign contacts, and then they added nine extensive new studies of their own. The new studies were conducted during the 2016 election with the labor group Working America, an affiliate of the AFL-CIO.

Their conclusion? Advertising and campaign contacts have almost no measurable persuasive impact, at least in general elections.

“The best estimate for the persuasive effects of campaign contact and advertising — such as mail, phone calls, and canvassing — on Americans’ candidate choices in general elections is zero,” Kalla and Broockman write. “Our best guess for online and television advertising is also zero, but there is less evidence” in these cases.

There are a few exceptions to this. The first is that persuasion is still possible in primaries, where voters aren't as tied down by partisanship. The study's authors say it's also possible to successfully target “pockets” of voters that have been identified as uniquely persuadable — such as abortions-rights supporters who were made aware of a Republican candidate's abortion opposition in a 2008 Senate race in Oregon. It also finds that persuasion is possible early in a general election, but that it wears off as you get closer to the election. This, the authors say, creates the illusion of persuasion actually having an impact.

The study's authors determined that the combination of all the experiments indicate that the "most optimistic estimate" is that campaign contacts persuade only 1 in 175 voters. Their own estimate is closer to 1 in 800 -- "substantially zero."

So that would suggest campaigns are basically a waste of time, then? Not exactly. Kalla and Broockman explain that it doesn't mean persuasion can't be effective at all — just that it might be time to rethink how campaigns attempt to do it. They also note that contacts can perhaps be effective for other purposes, like voter turnout:

To be clear, our argument is not that campaigns, broadly speaking, do not matter. For example, candidates can determine the content of voters’ choices by changing their positions, strategically revealing certain information, and affecting media narratives — dynamics which are outside the scope of our analysis but could be affected by advertising. Campaigns can also effectively stimulate voter turnout. Our argument is not that campaigns do not influence general elections in any way, but that the direct persuasive effects of their voter contact and advertising in general elections are essentially zero.

If candidates and leading campaign experts consume and actually agree with these conclusions, it could lead to a massive change how U.S. political campaigns are being run. The effectiveness of TV advertising is basically gospel at this point, and ad-buyers and media consultants are some of the most successful and well-compensated participants in the political process. The idea that their product can't actually persuade people will certainly meet with skepticism and resistance.

But the authors argue that their conclusions may not be too shocking to the experts. “This finding may help explain why campaigns increasingly focus on rousing the enthusiasm of existing supporters instead of reaching across party lines to win over new supporters,” the authors write.

Bingo. It's little secret that there are fewer and fewer persuadable voters in our increasingly polarized political process, but this study suggests that even the persuadable ones aren't being persuaded by ads and contacts. Those dollars might be better spent on turnout or other efforts.

Many of us scoffed at the idea that Donald Trump could win the presidency by appealing to 35 to 40 percent of the country and shunning the more traditional methods of campaigning — including TV ads and contacts. Perhaps he was just ahead of his time.