[Allan J. Lichtman is a professor of history at American University in Washington, D.C. His six books include Prejudice and the Old Politics: The Presidential Election of 1928 and The Keys to the White House.]

In 2002, Kathleen Kennedy Townsend, the Democratic nominee for governor in my home state of Maryland, declined to make a path-breaking choice for Lieutenant Governor on her ticket by tapping an African-American nominee. She instead chose a conservative white male. This decision drained the enthusiasm from her campaign. It cost her crucial support within the Democratic base vote and contributed to her upset defeat by RepublicanRobert Ehrlich in the general election.

Barack Obama, who is nearly the presumptive Democratic nominee, should not make the same mistake of choosing a conventional, white male running mate. Rather, he should complete the Democratic dream ticket by making Hillary Clinton his vice presidential choice. Likewise, if Clinton should pull off an improbable upset and gain the nomination, she should choose Obama as her running mate.

It is unusual but not without precedent for presidential nominees to tap a competing candidate as their choice for vice president.

In 1960, Senator Lyndon Johnson of Texas campaigned vigorously against Senator John F. Kennedy of Massachusetts for the Democratic nomination for president. The struggle continued to the convention, where Kennedy and Johnson took part in an unprecedented debate in front of the Texas and Massachusetts delegations. John Kennedy and Johnson didn’t especially like one another and Bobby Kennedy and Johnson detested one another. But Kennedy still chose Johnson as his running mate to put together a dream North-South ticket.

In 1980, conservative Ronald Reagan and moderate George H. W. Bush waged a bitter struggle for the Republican presidential nomination and the ideological soul of their party. Still, Reagan picked Bush as his running mate to unite his party, even though Bush had derided Reagan’s economic plan as “voodoo economics” and opposed Reagan on issues such as abortion and the Equal Rights Amendment.

I am not suggesting that the Democrats should put together their dream ticket in order to help the party beat John McCain. Given that the Republican opposition is suffering from an unpopular war, a sour economy, and a president with the highest disapproval rating in the history of scientific polling, the Democrats should be able to win with a vice presidential candidate plucked from the phone booth.

Rather, I think the Democratic dream ticket would be good for the party and even better for the nation. So far the intense primary contest has yielded many benefits for Democrats. Millions of new voters have signed up with the Democratic Party, Democratic primary turnout has hit record levels, and Democrats have attained their largest lead in decades in party identification. A ticket that includes both Obama and Clinton would help sustain this momentum and produce a record Democratic turnout in November.

The two candidates also appeal to different segments of the electorate. Obama is strong among African-Americans, young voters, and more affluent and educated voters. Clinton appeals to older voters, women, and blue-collar voters. Of course, some Clinton backers have said that they would not vote for Obama and vice versa. But those heat-of-the-battle sentiments will surely change once the general election campaign begins, especially if their first choice for president is on the ticket.

The Democratic dream ticket would also inspire young people and demonstrate convincingly that no one is excluded from the American dream of opportunity and success. The ticket might even contribute to expanding the representation of women and African-Americans in the second highest set of offices in the land: governorships and US Senate seats. At present there is but one African-American Senator (Obama) and two governors, including David Paterson of New York, who assumed the office after the resignation of Eliot Spitzer. There are only 16 women Senators and 8 women governors.

Six years ago in a small place called Maryland the Democratic Party failed to present the voters with a ticket that included both a woman and an African-American. Democrats can only hope that their party will not make the same mistake on a much larger stage in 2008.