WASHINGTON — John A. Boehner, the former speaker of the House, once stood second in line for the presidency and staunchly against legalized marijuana. Now you can find the longtime Republican standing before a wall-size photo of the Capitol, making an online infomercial pitch for the cannabis industry.

“This is one of the most exciting opportunities you’ll ever be part of,” Mr. Boehner says in an endlessly streaming video for the National Institute for Cannabis Investors. “Frankly, we can help you make a potential fortune.”

Mr. Boehner’s pro-weed epiphany coincides with the prospect of a payday as high as $20 million from the industry he once so vigorously opposed. He sits on the board of Acreage Holdings, a marijuana investment firm whose sale to a cannabis industry giant hinges on Mr. Boehner’s ability to persuade Congress and the federal government to legalize, or at least legitimize, marijuana.

The chain-smoking, merlot-sipping, former 12-term congressman from Ohio says he had never lit a joint in his life when he and the former Massachusetts governor William F. Weld, now a Republican candidate for president, joined Acreage’s board last year. This year, Acreage announced plans to sell itself to Canopy Growth, a Canadian company that is the biggest cannabis holding in the world. The deal, worth around $3 billion, based on current stock prices for both Acreage and Canopy, would create an $18 billion behemoth, industry analysts say.