Two kinds of candidates run for president: tinkerers and overhaulers.

Tinkerers look at America's problems and announce that with a tightening of a loose screw here and a squirt of Havolin Supreme there, the red-white-and-blue dream machine can be restored to full working order. A tinkering candidate is like the junior mechanic at the neighborhood body shop. He'll jack up your sputtering wreck in the morning and fix it by mid-afternoon. It was only ''the points,'' he says, or the brake fluid, or a front-end ''adjustment.'' A month later, it's a radiator leak or the back end. You'll be tinkered into bankruptcy before figuring out that the tightenings and squirtings have meant nothing.

Overhaulers, who are usually third- or fringe-party candidates, are frightening with their talk of replacing the engine and gears. Front end and back end are shot. The middle, too. Only the frame will stay, and that needs rustproofing.

Last week, the Democratic National Committee summoned candidates to Washington for two days of speeches. Cadences of tinkering were heard. ''We are here,'' said a DNC official to the gathering, ''to develop a positive message in order to reclaim the presidency in 1988 and beyond.'' Familiar crowd-noises of Democrats in convention were heard: denouncing of ''Republican country-club millionaires''; citing of polls that show support for more federal spending on the elderly, homeless and the environment.

No question exists that each of the candidates -- from the off-and-running Paul Simon to the off-and-mulling Pat Schroeder -- is qualified to be president. They have footing. They can stand firm in the crosscurrent of ideas and they have records of competence.

Despite this, they remain tinkerers, as do the others, from Michael Dukakis to Jesse Jackson. Dukakis, in a recent speech at Tufts University on ''creating a vibrant economy,'' showed that he is a junior mechanic of a governor looking for a promotion to the Oval Office garage. He said: ''We need a balance between research for military and civilian purposes; and in recent years it is military research and development that has dominated federal spending. This imbalance, if not corrected, will weaken America and weaken our national security.''

Will? It already has. Under Ronald Reagan, military spending, along with idolatry of military force as the guardian of security, has risen uncritically. Congress, protector of received orthodoxies, voted to support more than 90 percent of Reagan's military requests. Dukakis speaks of an ''imbalance'' that needs to be ''corrected.'' He meant it needs to be tinkered with and he has the Havolin Supreme.

He didn't discuss the overhaul required to convert the United States from a war-preparation to a peace-preparation economy. That might tar him with a label no Democrat in the race is apparently comfortable with -- the peace candidate.

No Democrat has brought up the idea of a true overhaul: unilateral disarmament. It isn't even on the agenda to get on the agenda. Jesse Jackson, a candidate with a record of being willing to take on any issue, avoids the appearance of going too far. He lists the weapons he opposes -- the B1 bomber, MX missiles -- but ends up promising to ensure a strong national defense. It's the code for announcing that he is safe. He can be counted on to tinker with military policies, not radically change them.

Tinkerers who come down with the fidgets when a radical idea passes between their ears forget that the White House has been occupied the last six years by an overhauler. Reagan offered himself as one in 1980. The voters said yes and he delivered. The military budget has increased 60 percent since 1981, weapons allocation by 300 percent. Money for low-income housing, food and children's programs has decreased. Tax benefits to the rich have increased, with the wealthiest Americans' payments dropping from 50 percent to 28 percent.

The Democratic tinkerers now becoming map-reading experts in Iowa and New Hampshire appear to be guessing that after Reagan's overhaul the country isn't ready for another one. It's R&R time. Dukakis, coming on as a competent manager, presents his proposals as ''idealism that works.'' It sounds like ''idealism that yawns.''