“Grassroots” movements in politics, by definition, are supposed to be natural and spontaneous.

So an ad that hit Craigslist on Monday certainly stood out. It sought participants for a “grassroots rally” in Prince George’s County against Maryland’s ballot measure on expanded gambling — and promised pay between $25 and $40 per person:

“One day, 1.5 hour project. You will be a rally participant. Duties are to stand in crowd, clap, cheer, hold a sign and be on time. Children are welcome. No further commitment. Payment at end of rally.”

The advertised rally was said to be taking place Tuesday morning in Upper Marlboro.

Sure enough, an anti-gambling rally is scheduled Tuesday in Upper Marlboro. But the organizers claimed to have nothing to do with the ad — and no one else exactly stepped forward to take responsibility either.

“We’re not paying anyone,” said Bishop Joseph H. Thomas, chairman of the Community Empowerment Coalition, which issued a press advisory Monday for a “grassroots community rally press conference.”

Thomas’s organization has long opposed plans to allow a new casino in Prince George’s County, where National Harbor is considered the most likely location.

Question 7, which will appear on Maryland’s ballot in November, would allow a casino in the county, as well as Las Vegas-style table games at the state’s five existing slots locations.

Thomas vowed to look into who is behind the ad. Another version — which promised top pay of $45 instead of $40 — appeared on the Web site ClassifiedAds.com. The job it listed as available: “Rally participant — Crowd Filler.”

Both versions of the ad provided the same email address for those interested in applying. An inquiry by The Washington Post requesting additional information went unreturned early Tuesday evening.

So, too, did a call placed to a contact person listed on the ClassifiedAds.com.

Question 7 has attracted massive amounts of money from companies with a stake in the outcome of the expansion plan. As of Monday, the two sides together had reported giving more than $34 million to ballot-issue committees set up to campaign for and against the measure.

Penn National Gaming, the only funder so far of the opposition, has itself given more than $18 million.

A spokesman for the Penn-funded ballot-issue committee said his group had no role in organizing Tuesday’s event and that he was even unaware of the location.