Returning to Washington and the ceaseless combat with Congress, President Carter will need all the peace of mind that he picked up on his vacation. The battle lines are drawn as they have been from almost the first weeks of his presidency.

He may gain a point or two - his veto of the defense-appropriation bill over the $2 billion nuclear powered aircraft carrier sustained - but the fundamental impasse will remain. As with the energy bilL, it concerns inflation, the decline of the dollar, the confusion over foreign policy, the future of this country.

Carter has been the favorite whipping boy. It may be significant, however, that in the latest Harris Poll the negative rating for Congress is only slightly lower than that for the president. The two branches of government stand low in the esteem of the nation.

And that seems to speak to the system itself, and it seems almost heresy to say that our system of divided powers is not the best in the world. A great many faults can be found with the Carter presidency. But if he is contesting with a system that is demonstrably unworkable, it may be short of justice to pile the blame on the man occupying what Harry Truman called the impossible office.

We have seen what can happen. In 1930, with the onrush of the greatest depression ever, Herbert Hoover was deadlocked with the Congress, with no action possible. In the process Hoover, a man of exceptional ability if somewhat rigid views on the economy, was destroyed, and the country hit bottom with all the blanks closed.

So here's for some heresy. Would a parliamentary system serve us better? Under such a system, in theory at least, when a vote on a major issue goes against the government in power, that government falls. Elections are held in a short time, and the voters can pass on the question that brought the government down. Thus, there could be no protracted deadlock on, say, an energy program.

The paradox, of course, is that there is a large Democratic majority in both houses of Congress. Among the many naive promises he made during his election campaign, Carter said that a Democratic president could work happily with a Democratic Senate and House.

Already political reporters are conjecturing about 1980, with a presidential election more than two years off. We talk about candidates - who's up, who's down, who's on third. This assumes that a new face in the White House will in some fashion mean a magical transformation so government can function.

Even at this early date the New Hampshire primary comes into the speculation. There the votes of a few hundred thousand Republicans would presumably set the pace. Gov. Meldrin Thompson, glowering at all interlopers like an angry bull, is determined that the prize will go to his candidate, Ronald Reagan.

The best guessing is that Reagan, who has put such formidable effort since 1976 into the contest for Republican delegates, will be the nominee of his party. Take it a step further and say that, with the country going to the right, Reagan might win the election, defeating, say, Carter emerging from an unruly contest in his own party or, possibly, Sen. Edward M. Kennedy (D-Mass.).

Can anyone believe that such an outcome would mean orderly government? The makeup of the Senate and House could not conceivably change within a span of three or four years to match Reagan's policies. So the prospect would be for deadlock again, with the game of veto and override played from week to week.

It may be said that a Reagan or a John Connally, another horse in the race, would provide stronger leadership than has Carter. But this, too, seems to me a myth. Leadership is a word invoked to inspire hope that the unworkable will work.

One question is whether anyone in a quarrelsome and fragmented Congress might come up with a proposal for making the system responsive to the needs of the country and the world. Is there a young man with the courage to say that the emperor has no clothes? The odds are against it.

Just before his last illness, Walter Lippmann was working on a book with the tentative title, "The Ungovernability of Man." I wish he had lived to bring his great wisdom to the core of our dilemma today.