In presidential politics, the process of picking a running mate can be boiled down to four words: First, do no harm.

When Sen. John Kerry, D-Mass., chose Sen. John Edwards, D-N.C., for the No. 2 spot last week, Democrats responded with enthusiasm. But few, if any, selections have helped win the White House. And some running mates have certainly dragged their tickets down.

Surprises backfire, scholars said. In general, selecting a known quantity is the way to go, said Michael Nelson, who has studied the vice presidency.

The worst picks have been made by candidates “who thought they were doing something really politically shrewd, and it turned out they were doing something really stupid,” he said. “The presidential candidates got too clever by half.”

The question: Which politicians were rotten running mates?

The experts:

Dan Coen, author of “Second String: Trivia, Facts and Lists About the Vice Presidency and its Vice Presidents.”

Michael Nelson, political science professor at Rhodes College in Memphis, Tenn.

Timothy Walch, editor of “At the President’s Side: The Vice Presidency in the Twentieth Century” and director of the Herbert Hoover Presidential Library-Museum.

Their picks:

Spiro Agnew: A surprise selection, Richard Nixon’s running mate was assigned the role of attack dog. Once in the White House, Nixon reportedly quipped that the prospect of Agnew becoming president was his best insurance against assassination. In 1973, Agnew pleaded no contest to tax evasion and resigned.

Thomas Eagleton: George McGovern selected the senator from Missouri in 1972 and then dumped him after it was disclosed that Eagleton had been treated for depression and had received electric shock therapy. Sargent Shriver took his spot, and the Eagleton episode spurred more intense vice-presidential vetting in future years.

Hannibal Hamlin: Abraham Lincoln’s vice president found the job dull, and he seldom visited the White House. Hamlin deemed himself the most unimportant man in Washington and enlisted in the Coast Guard. He was dropped from the ticket during Lincoln’s 1864 re-election campaign.

William King: The senator from Alabama ran on Franklin Pierce’s ticket, but his declining health kept King from campaigning much. He lived with James Buchanan for years, and critics called him “Buchanan’s wife” and “Miss Nancy.” King took the oath of office while in Cuba and died from tuberculosis only weeks later.

Bill Miller: Barry Goldwater tapped a nearly unknown New York congressman as his running mate in 1964. The presidential candidate explained the decision by saying that Miller annoyed Lyndon Johnson.

Dan Quayle: Mocked as an intellectual lightweight, Quayle quickly became the butt of political jokes. He famously criticized a television character for choosing to become a single mother and added an extraneous ‘e’ when he spelled the word “potato.” Many suggested that President George Bush should have replaced his running mate in 1992, but he stuck with his pick during his losing campaign against Bill Clinton.

The best

Vice presidential experts count these vice presidents as among the most accomplished: Theodore Roosevelt, Harry Truman, Lyndon Johnson. Also on the list were Al Gore, for reinforcing Bill Clinton’s strengths, and Dick Cheney, because he has no political ambitions of his own, and he raises buckets of cash.