President Obama said at a press conference that the 2016 presidential election isn't rigged, responding to an accusation made by Republican nominee Donald Trump, on Aug. 4 at the Pentagon. (Reuters)

With nearly three months to go before the presidential election is decided, Donald Trump is already explaining his loss.

Trump has argued with increasing certitude that it would be due to a “rigged” system that includes voter fraud on a massive scale — all apparently to the benefit of Hillary Clinton. It also apparently won't matter how badly he loses; fraud and rigging will account for the margin.

“The only way we can lose, in my opinion — and I really mean this, Pennsylvania — is if cheating goes on,” Trump said Friday night in Pennsylvania, a state where the cheating would have to account for Clinton’s current double-digit lead in the polls. “I really believe it.”

The question is how many Republicans will believe it. And based on the data we have, the answer is: Probably a lot.

The conspiracy theory is brazen in its simplicity and lack of factual basis, especially this far before votes are actually cast. And yet, in a number of ways, it’s pretty clear that this is the kind of explanation that many Republicans will embrace.

A Bloomberg poll released last week showed that 34 percent of likely general election voters agreed with the statement that the election will be “rigged” — a number that included 56 percent of Trump supporters.

The question, it bears noting, was more about general rigging of the election and not specifically whether it would actually be enough to tip the scales in one direction or another.

The Democratic-leaning automated pollster Public Policy Polling asked North Carolina voters last week a more specific question: “If Hillary Clinton is elected president, do you think it will be because more people voted for her, or because the election results are rigged for her?” This is establishing a more direct causal relationship; it’s not just the election being "rigged,” but also that this rigging will be the deciding factor. In this case, 69 percent of Trump supporters agreed.

Both of these polls could oversell just how much Trump backers will actually blame a rigged system for a Clinton win. Each offers a binary choice in which the alternative to a rigged system perhaps just isn’t attractive — the idea that the system isn't rigged and that Clinton will win more votes. But, clearly, many and even a majority agree that some kind of rigging exists.

And the specific idea of a rigged 2016 election aside, a large number of Republicans have embraced these kinds of conspiracy theories and voter-fraud allegations in the past — even in the absence of any real evidence.

Trump himself, of course, once spearheaded the effort to question whether President Obama was actually born in the United States and was a legitimate president. This movement has died down a bit in Obama’s second term, after he showed his Hawaii birth certificate, but a September CNN-ORC poll showed that 20 percent of Republicans still volunteered that Obama wasn’t born in the United States and 12 percent had no opinion. Similarly, in a PPP poll that month, 29 percent of Republicans said Obama wasn’t born in the United States.

The unfounded idea that Obama is a Muslim is even more widely embraced. The CNN poll showed that 43 percent of Republicans said Obama was a Muslim (just 3 in 10 correctly said he's a Christian), while PPP pegged that number much higher — at 54 percent.

Trump’s idea of a rigged election seems to be grounded — to the extent that he has explained it — in the kind of voter fraud that could move election results several percentage points or more in many states. There are exceedingly few instances of proven voter fraud in the United States and nothing even close to the scale that Trump is talking about, but GOP voters do appear to believe that such things are possible.

A Monmouth poll on the eve of the 2012 election showed that 51 percent of Republicans believed voter fraud to be a “major problem” in the United States. Similarly, a Washington Post-ABC News poll in July 2012 showed that 57 percent of Republicans thought voter fraud was a major problem.

Those are pretty general sentiments, but a Marquette Law School poll in Wisconsin in 2014 broke it down a little more (albeit only among Wisconsinites). Given a choice between how many votes were affected by at least one type of voter fraud, 54 percent of Republicans picked the largest option, saying at least one kind of fraud affected a few thousand votes or more in each election in that state. And 41 percent of independents agreed.

A few thousand votes in Wisconsin, it bears noting, accounted for only about 0.1 percent of the vote total in the 2012 election — not the several points that would have to move for Trump's theory to hold water. He trails by double digits or close to it in many swing states. So this isn't the same as saying that Wisconsin Republicans believe in voter fraud on the kind of scale Trump is talking about.

We don’t know whether Republicans believe in the kind of fraud that can move results five to 10 percentage points for one simple reason: because it has never been seriously alleged by a major political candidate, and, thus, it hasn’t been polled.

Trump’s allegation strains credulity in many ways. Even if you believe voter fraud is more prevalent than has been proved, he’s predicting that hundreds of thousands and even millions of votes across the country will be affected by voter fraud and it will be enough to change the result — before it even happens.

But the ground in which Trump planted this particular seed is indeed fertile.