The seven Democratic presidential candidates (Bruce Babbitt, Joseph Biden, Michael Dukakis, Richard Gephardt, Albert Gore, Jesse Jackson, Paul Simon) are getting their acts together and taking them on the road, and the road recently took them to Houston for a sort of New Haven opening for their (they hope) Broadway-bound shows.

Host Bill Buckley began by saying that President Reagan has five portraits in one room -- of Jefferson, Lincoln, Taft, Coolidge and Eisenhower -- and asking each candidate which portraits he would replace.

Jackson said he would replace the portrait of Hoover. The portrait isn't there, but Jackson is a traditional Democrat after all. He understands this rule: First, bash Hoover.

Gore stroked the party's obvious erogenous zones (Wilson, FDR, Kennedy), but added that he also wanted to hang the portrait of James K. Knox (Gore meant Polk) because he was a dark-horse winner from Tennessee. Gephardt allowed as how no portrait could be as pretty as a framed copy of the Constitution. Simon said he would have portraits of jus' plain folks. Then, alarmingly, he began to list the categories -- steelworker, coal miner, family farmer (agribusiness need not apply), working mother, teacher, inner-city schoolchildren. Amazingly, and mercifully, he stopped there.

Biden viewed with alarm the fact (or so he says) that ''75 to 80 percent'' of MIT's ''students'' are working on defense projects rather than commercial applications of science such as ''how to make automobiles better.'' Biden has a low-tech notion of the proper use of MIT. Biden said something sensible -- that elimination of all unfair trade barriers would still leave America with an enormous trade deficit -- but then said that a proper president would be ''insisting'' that American management and labor do their jobs better. Biden is saving for later the details about the practicalities of presidential ''insisting.''

Jackson, showing mid-season form (as you would expect from a campaigner who knows no off-season), got off a characteristic trope, deploring ''the purging of our jobs and the submerging of our economy.'' Gore, the barefoot boy from Harvard, gave Jacksonspeak a game try, endorsing ''perfectionism, not protectionism.'' Gore sounded like a chamber musician taking a fling at rap music.

There once was an Indianapolis concert featuring 50 pianos. Pianos are nice, but so is a little leavening. The Houston debate seemed long on sameness. The candidates are all dressed up and ready to run against Reagan and Star Wars, and to be foursquare for competitiveness and being nice to children.

A few days after the debate, Richard Scammon, a student of the tonalities of our politics, was musing about tones not heard in Houston. Perhaps, he said, the two best candidates for the two parties are Florida's former governor and current senator, Bob Graham, a Democrat, and California's Republican governor, George Deukmejian.

Graham is Sunbelty as can be; he is from the fastest growing state; he is from the South but is not too southern; and obviously wins elderly and Jewish voters. Because of the Cubans and Nicaraguans in southern Florida, Graham can vote for (and liberals can forgive his votes for) aid to the contras.

Deukmejian is not just ethnic, he is Armenian. Jackpot: the Turks massacred the Armenians, so Greek-Americans will support him. In terms of parsimony and hostility to taxes, Deukmejian is California's first really Reaganite governor.

Stephen Ambrose, biographer of Eisenhower and now Nixon, notes that in the last nine presidential elections, Democrats have won two (1964, 1976) and, in terms of the popular vote, essentially tied one (1960). In the tie, and in all six the Republicans won (1952, 1956, 1968, 1972, 1980, 1984), there has been a Californian on the Republican ticket and California has voted Republican. Republicans lost in 1948 with a Californian -- Earl Warren -- as vice presidential nominee, but California was then just achieving critical mass. Democrats have never nominated a Californian for either spot on their ticket.

Graham and Deukmejian are not on the menu so, to be serious, let's deal with the Earlobe Theory of History. Simon scored well with viewers of the debate, but a political consultant says, ''He may have the highest IQ, but he's never going to photograph well. . . . His earlobes are unbelievable.'' The consultant clearly thinks the point is as plain as the nose on Cyrano's face.

In the new movie ''Roxanne,'' Steve Martin's version of ''Cyrano,'' a newcomer in town is warned not to stare at Martin's large nose. But he stares. Then he gets over it. And Martin gets the girl, a fact that should sustain Simon's patience until people stop staring.