Nearly two weeks ago, multimillionaire commodities trader Richard Dennis flew into Washington for a rare meeting with the president of the Roosevelt Center, the think tank he has bankrolled. He walked into the office of Douglas J. Bennet, the president, and asked him to resign.

In the rarefied atmosphere of Washington's think tanks, where political battles are usually studied and not engaged in, Dennis' action stunned and angered much of the Center's staff. Bennet was a respected and well-liked president. What had gone wrong?

The dismissal was pure Richard Dennis--the trader with the "contrarian" logic--the skill of knowing when to act contrary.

When Dennis set up the Roosevelt Center for American Policy Studies a year and a half ago, he staffed it with people of varying political alliances. He had a different kind of think tank in mind. In a flotilla of Washington institutions with discernible political bents, the Roosevelt Center would be a brave tugboat of nonpartisan, nonadvocacy thought. Unlike the others who seemed to write for each other, it would reach out and make itself heard beyond the Beltway.

The Center--staffed by about 25 people--thought it was doing just that. Its fellows had compiled educational packages, set up debates and bipartisan study committees, and made videotapes on current issues.

"We don't sit here and write books," said one Center official.

But Richard Dennis was not satisfied. Bennet, the man who has been hurt the most, is the one who seems to understand it best:

"What we tried was a high-risk relationship," said the unflappable Bennet. "It didn't work. So be it."

Yesterday, the Center's board met in Chicago, Dennis' hometown and business base, and installed Charles P. Wolff as acting president of the center. Wolff is the Center's chief operating officer and Dennis' aide in Chicago. (For at least a week, speculation abounded that board member Roger Molander, executive director of Ground Zero, a nonpartisan group set up to educate people about nuclear war, would hold the interim title. But some Center officials worried that Molander's association with Ground Zero--nonpartisan though it may be--would scare off the moderates and conservatives working on Roosevelt Center projects.)

On its most intriguing level, the brouhaha provided a glimpse of Richard Dennis, the consummate outsider, clashing with Douglas Bennet, the consummate Washington "public policy entrepreneur"--as he chose to describe himself.

"They come from completely different cultures," said one person close to the Center. "They must have been talking past each other these past 18 months."

"Well, I guess what I would say is that we are two very different people with two very different agendas in my Roosevelt Center," said Richard Dennis yesterday by phone from Chicago. "I don't want to characterize his, but mine is more interested in the policy world outside Washington than his is... I think we did a good job in Washington. I think we can and should do a better job outside Washington."

Bennet, 43, a former director of the Agency for International Development, a Harvard PhD in history, a man who worked for Vice President Hubert Humphrey, grew up in a politically active family. "He understands the political process," said the source. "His whole style is to work quietly on the inside."

Dennis, on the other hand, is described by some at the Center as an eccentric loner who has "tremendous difficulty communicating with people." Said the source, "He has worked outside the system his whole life. He's been extremely successful by taking risks."

"My record is one of risk-taking, too," said Bennet. "I've run for office three times and lost, I've set up three institutions... I do understand Washington and how it works. What I do must look as arcane to Dennis as what he does looks to me.

"There was only one requirement to make this combination work. Communication. And there hasn't been any."

Dennis denies that he didn't make himself clear in his dealings with Bennett. "He's known of my feeling that we could be doing different things for about a year."

Dennis says he wants people outside Washington to have a vehicle for dealing with the federal city. "I can't give you a blueprint of how this would happen... but people outside Washington don't know how to have any impact on public policy."

He, himself, was one of those people.

"Absolutely," he said. "I make no bones about that. The difference is that I had the abilities to do something on this front."

One person at the Center said that Dennis found Bennet "condescending." Dennis said, "No comment."

Dennis visited his center only about half a dozen times in the past year. Located on the fifth floor of 316 Pennsylvania Ave. SE, it occupies a modern suite of white-walled offices, appointed simply with contemporary furniture and potted trees.

Dennis has pledged $5 million for the Center over three years, and has already put in over half that.

Money is no problem here. Not for Dennis, who just turned 34 and was a multimillionaire by 25. According to the Dec. 6 issue of Business Week, market insiders say he amassed profits of $40 million to $50 million in trading in 1982 alone. Dennis does not dispute this.

He is described by people at the Center as a shy man. "I used to tell people that Richard wore prewrinkled clothes," said one Center official. "He is not elegant . . . His interest is in the game of making money, not accumulating it."

"I spend about twice what everyone else does and half what people with my money would," said Dennis. He rents an apartment on Chicago's lakefront Gold Coast, and owns homes in Wisconsin and Florida. He drives a 6-year-old Mercedes. "It's wrinkled, too," he said.

He has given generously to Democratic political candidates and some Republicans. He contributed $50,000 to Democrat Adlai Stevenson's unsuccessful race for the Illinois governorship last year.

When Dennis started the Roosevelt Center--which also has a small office in Chicago--he wrote in the brochure:

"Amidst the clamor of special interests, someone must champion the public interest . . . As a nation carrying worldwide responsibilities, we must start caring enough about our future to make farsighted choices, beyond partisanship and ax-grinding."

The people he hired say they could not have agreed more with his outlook. They set about fulfilling it.

Douglas Bennet, hired at more than $100,000 a year, decided the office should be on Capitol Hill at the heart of policymaking. "Doug didn't want us on Massachusetts Avenue downtown with other think tanks," said Center fellow Norman Sherman.

Center fellow Nelson W. Polsby, a political scientist, suggested that to encourage collegiality there be no doors on the offices. The Center compromised by making the doors predominantly glass.

The Center set up committees to draw experts from disparate viewpoints into discussion. Sens. Sam Nunn (D-Ga.) and John Warner (R-Va.), for instance, chair a bipartisan group on reducing the risk of nuclear war. Frank Carlucci, a former Reagan deputy secretary of defense, and W. Harrison Wellford, the former executive director of OMB under Carter, chair another on the status of public servants.

The Center sponsors a column in a large national circulation (800,000) weekly called Grit. It has produced "issue" videotapes and for eight months last year, sponsored a cable television series on C-Span called "The American Debate" every Sunday night.

"As far as I'm concerned, we've been a roaring success," said Sherman.