At age 74, Robert J. Roosevelt is the oldest candidate trying to succeed Rep. Michael D. Barnes. He may also be the most colorful.

His campaign speeches have as their themes such subjects as conspiracy theories, allegations about a worldwide network of Italian terrorists and calls for the impeachment of President Reagan.

The other Democrats in the primary field have paid scant attention to Roosevelt, who has run twice for the U.S. Senate and twice for the 8th District seat in Congress, each time without success.

"I know more about what to do than the other candidates," Roosevelt said in a telephone interview from his Bethesda apartment. "The other candidates are all involved in heavy corruption of one form or another."

Roosevelt, a California native who has lived in this area for many years, is a former construction worker who is a researcher of national political issues. He said he has raised no money for his electoral effort.

Although he was not born to the Roosevelt political dynasty -- he said he changed his given name, Robert Grant, more than a decade ago -- the candidate maintains that he is the great-great-grandson of Theodore Roosevelt.

In 1974, when he was one of 10 Democrats vying for the chance to challenge Sen. Charles McC. Mathias Jr., Roosevelt told reporters that he had traced his family tree to Benjamin Franklin, William Penn and King John of England.

His slogan in that primary election was, "Elect an American for a Change." Roosevelt finished far back in the pack, receiving 10,175 votes, 3.2 percent of the statewide total.

In the spring of 1976, after Republican Gilbert Gude announced his retirement from the 8th District seat, a dozen Democrats, including Roosevelt, vied for their party's nomination. Roosevelt finished in 10th place, with slightly more than 300 votes.

Two years later, Roosevelt ran for the House again, finishing next to last (2,245 votes) in a field led by Barnes. Barnes went on to unseat Newton I. Steers.

In September 1982, Roosevelt launched another bid for statewide office, this time against Democratic Sen. Paul S. Sarbanes. One of Roosevelt's most memorable lines in the campaign was calling the French Navy "possibly the worst nuclear menace in the world."

The candidate received 8,668 votes statewide but lost.

Roosevelt, blaming his defeats on his failure to campaign actively, has attended a couple of Democratic Party forums, where he has made a number of accusations. He maintains, among other things, that Libyan leader Muammar Qaddafi is an Italian terrorist, that the entire staff of the Metro transit system is corrupt, and that a Montgomery County grand jury is looking into the affairs of all Democrats running for the 8th District seat.