Warning darkly of a stolen election, Donald J. Trump has called on supporters to turn out in droves on Election Day to monitor polling places, telling them they need to be vigilant against widespread voter fraud and a rigged outcome.

“Voter fraud is all too common, and then they criticize us for saying that,” he said at a rally Tuesday in Colorado Springs. “But take a look at Philadelphia, what’s been going on, take a look at Chicago, take a look at St. Louis. Take a look at some of these cities, where you see things happening that are horrendous.”

His language has stirred increasing fears of intimidation of minorities inside polling places, where their qualifications to vote could be challenged, or outside, where they would face illegal electioneering.

But as Mr. Trump casts doubt on the integrity of the presidential election, there are no signs of a wave of Trump poll watchers building. Like much else about his campaign, his call to “get everybody to go out and watch” the polls seems to be a Potemkin effort, with little or no organization behind it.