Donald Trump knows this much: If you say something often enough, people will believe it.

So the Republican presidential nominee has been insisting, over and over, that the election is "rigged" -- that the "elites," backing Democratic nominee Hillary Clinton, are "stealing" the election.

This charge -- meant to undermine America's democratic institutions, all so Trump has an excuse in case he loses -- is taking root. It doesn't matter that there is zero evidence that the election is rigged, or that the Democrats or the elites or whoever has the ability to "steal" the election, seeing as each state is responsible for administering elections in its jurisdiction. (Thirty-one states have Republican governors.)

A new Politico/Morning Consult poll found that 41 percent of voters surveyed believe the election might be stolen from Trump because of "widespread voter fraud." Some 75 percent of Republicans hold this view. Seventeen percent of Democrats believe a stolen election is possible.

This is not a new strategy for Trump. Whenever data proves inconvenient (in this case that polls show Democratic nominee Hillary Clinton widening her lead over him), he attacks the legitimacy of the source. During his campaign, Time magazine reports, he has attempted to "sow doubt about the Bureau of Labor Statistics, the Federal Bureau of Investigation, the Federal Reserve, the Commission on Presidential Debates, scientists studying climate change, the news media" -- and now the election process.

Trump, who on his ballot biography lists as an achievement having "over 20 million followers" on social media, has called for his supporters to go to polling stations to serve as poll watchers, raising the possibility of widespread voter intimidation.

The Politico/Morning Consult poll suggests that if Trump does lose the election on Nov. 8, millions of Americans might not believe the result is valid, further eroding trust in government and further emboldening extremist groups.

CNN media correspondent Brian Stelter, for one, believes the press isn't taking Trump's "rigged election" talk seriously enough.

"We as a country cannot allow ourselves to become numb to this," he said on air over the weekend. "We as a media cannot shrug it off as old news ... Some of his supporters believe him."

-- Douglas Perry