After six months in office President Carter has established unchallenged personal supremacy in Washington. He has set his mark on both foreign and domestic policy in ways that command robust public support. But he has given short shrift to the most critical domestic problem, the problem of the cities, while mishandling the most important foreign problem - relations with the Soviet Union.

THe hallmark of the administration to date is the President's personal ascendancy. He is the administration's chief strategist and tactician, diplomat, economist, congressional lobbyist and public-relations man. Except for a handful of persons who enjoy his special favor - notably Vice President Walter Mondale. Press Secretary Jody Powell. Political Adviser Hamilton Jordon - nobody has emerged as much of a figure in Jimmy Carter's Washington.

Inevitably the President's dominance has made it impossible for cabinet officers, despite talk of their more important role, to come into their own. The strongest of them - energy czar James Schlesinger. Defense Secretary Harold Brown. Treasury Secretary Michael Blumenthal and HEW Secretary Joseph Califano - have been obliged to accept Carter deadlines they knew to be impractical.

They and others have been diminished by balance-of-power games. Secretary of State Cyrus Vance, the true foreign-policy professional in the crowd, is offset by Brown, who has a passion for technical progress, and Zbigniew Brzezinski, the National Security Adviser, who has it in his head to make history by outdoing Henry Kissinger. Blumenthal and Charles Shultze of the Council of Economic Advisers have had their activist tendencies neutralized by Bert Lance, the conservative banker-cum-politico who serves as Director of the Office of Management and Budget.

Precisely because the President is so much in charge, and the representatives of bureaucratic interest is so little, the Carter administration remains close to the electorate. There is a tighter link between policy and politics than in any previous administration I have seen.

Domestic policy fits exactly the post-Watergate mood of the country. The emphasis has been on brisk administration and clear-cut decisions. Great attention has been lavished on a comprehensive energy program, on balancing the budget, on maintaining environmental standards, on government reorganization, on tax reform and on ending pork-barrel water projects. These measures are so much in tune with majority opinion that the Congress, while fencing on details, is going along without anything like the fuss so widely predicted.

But the one economic measure specifically designed to help poor people - the $50 tax rebate - was withdrawn. No important new programs have developed for meeting the agonizing problem of the cities. I see no reason, after six months, to change the judgment, rendered after three months that in economic policy the Carter administration is the most conservative Democratic regime in this century.

In foreign policy, the national narcissism that set in following the defeat in Vietnam has been faithfully reproduced. By emphasizing human rights, public diplomacy, arms reduction and the American role as soother of troubles in Africa and the Mideast the Carter administration has catered to the image of a great and good country.

A few positive benefits have flowed from the new emphasis. Efforts for quick settlement of all disputes with Panama. Vietnam and Cuba have been set in motion. The U.S. is no longer associated with the sure losers in Africa, nor in an untenable straddle on dealing with the underdeveloped world. The elements of a Mideast settlement have been laid bare. There is now a better chance that Israel and the Arabs will go to a Geneva Conference - though equally that the Geneva Conference will get nowhere.

With Russia, however, the new policy has clearly not worked. There is small prospect of reaching an arms-control accord before the present agreement expires on Oct. 3. Relations between Washington and Moscow, and Carter and Brezhnev, have gone sour, and their sourness is now curdling relations with major allies. Worst of all, Washington seems to have no ideas for setting matters right with the Russians.

So far, to be sure, the crowd-pleasing approach of the administration has almost totally eclipsed the perilous consequences that may ensue. An advancing economy at home and an unusually calm period in foreign policy have fostered an impression that all's well. Perhaps it is, but perhaps, too, it is a good thing that that proposition has not yet been tested in a serious crisis.