I haven’t had much good news for Bernie Sanders supporters this year, so let me go out of my way to provide some today: There are very good reasons to question a new Monmouth poll that shows Hillary Rodham Clinton leading by 41 percentage points in Iowa.

Most other polls have tended to show a tight race, but Monmouth shows a blowout. Why?

One possibility is that Mrs. Clinton has made gains over the last few weeks, thanks to Joe Biden’s withdrawal from the race or her performance in the Benghazi hearing. But another possibility is the sampling frame of the survey. There are good reasons to believe that the Monmouth poll excludes many voters who are supporting Mr. Sanders.

What’s a sampling frame? Basically, it’s the people who could be selected to participate in the survey. According to the Monmouth poll’s methodology description, the poll’s sample was drawn “from a list of registered Democratic voters who voted in at least one of the last two state primary elections.”

These two conditions — being a registered Democrat and recent primary participation — exclude many of Mr. Sanders’s supporters.