Tony Coelho is so good at what he does, he's scary. His success tells a lot about what's right -- and what's wrong -- with the Democratic Party.

Coelho is the 43-year-old congressman from Merced in California's Central Valley and the chairman of the Democratic Congressional Campaign Committee. He is an appealing and courageous man, a dynamic overachiever who has publicized his own battle with epilepsy as an encouragement to others who have that disease.

Politically, he has done so well since taking over the election committee for House Democrats that he is the front runner for election as party whip in the next Congress. That is the No. 3 leadership position and, traditionally, a steppingstone for future speakers.

Coelho's goal, he told me last week, is to see that not one Democratic House incumbent is defeated in November. If that seems far-fetched, even for a man of his ambitions, consider this: in 1982, the last nonpresidential year, only three Democratic incumbents lost House seats -- two of them because reapportionment forced them to run against Republican colleagues in Republican-leaning districts and only one to a nonincumbent challenger.

Coelho has spent his adult life learning the ways of the House and is a master of its politics. For 15 years, he served as an aide to Rep. Bernie Sisk, his district's congressman, and succeeded Sisk when he retired in 1978.

Since he took over the Democratic Congressional Campaign Committee in 1981, Democrats have picked up a dozen seats. He has transformed the committee from a pale shadow of its Republican counterpart into an effective fund-raising machine and has built a modern media center for Democrats to distribute their electronic messages.

Coelho's great skill is helping fine-tune campaigns. His official biography notes that earlier in his career he was a consultant to the House Parking Committee. He is as adept at fitting issues to districts as he was in finding spots in the House garage that suited members' needs.

He can tell you in one sentence how Democrats can exploit public fears about safety in airlines, drugs and food to make the case for activist government and, in the next sentence, brag that a Democrat in Nevada is winning because ''he's running against the government.''

Since it is an article of faith in the House that ''all politics is local,'' Coelho's tactical genius makes him many allies. The difficulty arises when tactics begin to control policy choices. Coelho discovered in a hard-fought special congressional election in Texas last year that a tough line against foreign imports stirred the voters. Ever since, he has pushed hard for House Democrats to take what he calls ''an aggressive stance'' on trade issues.

The highly restrictive trade bill that passed the House last month in the face of veto threats from President Reagan was a central piece of Coelho's strategy for the November elections. He is not fazed when that bill is denounced by editorialists, who are rarely in tune with the Reagan administration, as a dangerous piece of protectionism. The trade issue ''worked'' for the Democrats in Texas last year, Coelho says, and he can tell you a dozen specific districts from Maine to North Carolina to California where it may help swing seats to the Democrats this year.

As a tactician, he rejoices that, ''We've put the Republicans on the defensive.'' Whether the legislation is ''responsible'' is another question. ''Our bill won't become law,'' he says, as if that were the answer to the objections. ''We're forcing the administration to react to the problem of lost American jobs, and that's being responsible.'' The most visible administration response to passage of the House trade bill was a sudden move to shut down imports of Canadian cedar shakes and shingles. Canada in turn has taken angry retaliatory action against American computers, semiconductors, books and magazines. Two weeks after the Democrats' trade bill passed, there is talk of a trade war between the United States and its largest trading partner.

Canadians do not vote in American elections, and the long-term damage to the United States' economic future from indulging in short-term protectionism will be a negligible factor in this fall's congressional races. Tactically, Coelho is surely right that it's far better to tell your constituents how ''tough'' you are on foreign traders than to deal with the root causes of the upheaval in the international economy.

It is the triumph of Tony Coelho to make the Democratic message sell so well in 250 separate districts that not one incumbent may lose. It is the tragedy of the Democratic Party that in message and meaning, its whole is so often less than the sum of its parts.