A few years ago, Paul Manafort was a disgraced political operative living in a windowless cell. If Donald Trump wins in November, Mr. Manafort is likely to re-emerge as one of the most powerful people in Washington.

Because of Mr. Trump’s transactional nature and singular method of wielding power, as president, he would probably empower a small group of lobbyists who could profit from their access. Though no one elected them, these gatekeepers could exercise sweeping influence over U.S. policy on behalf of corporations and foreign governments, at the expense of regular Americans who can’t afford their services.

Rather than drain the swamp, an unleashed President Trump would return the lobbying industry to the smoke-filled rooms of the 1930s, an era unchallenged by the decades of reforms since Watergate.

And Mr. Manafort, whose career has been based on lobbying the same people he helped put in office, would be at the center. “A new Trump administration would be a bonanza for Paul,” says Scott Reed, a Republican political strategist who hired Mr. Manafort to work on Bob Dole’s 1996 presidential campaign. “Trump is the Manafort model: access at the highest levels for his clients and friends.”

A second Trump term, with the likelihood of yes-men and lackeys having more sway than political professionals and civil servants, would all but return Washington to an era when the nation’s laws were negotiated over steak dinners and golf. In the early 1970s, the leaders of a U.S. tool and die company worried about losing a Defense Department contract. They met with the era’s top lobbyist, Tommy Corcoran, who had worked in the White House for President Franklin Roosevelt and later advised Presidents John Kennedy and Lyndon Johnson.

Mr. Corcoran picked up the phone and called a Pentagon contact. After a brief exchange, he hung up. “Your problems are over,” he told his new clients. His $10,000 bill is roughly the equivalent of $75,000 today.