WHEN President Carter took office, he said that energy policy would be the most difficult challenge to his adminstration. That's been true -- and it is likely to be equally true in the next administration. A failed energy policy can demolish any hope for stable economic growth, jobs and the social progress they bring. The danger of serious disruption in the oil flow is not hypothetical -- as the war between Iran and Iraq demonstrates.

On energy, the contrasts among the candidates are sharp and clear. The Carter policy is on the right track, although it doesn't go far enough to give the country the protection it now needs against sudden cutoffs abroad. The Reagan position is merely to trust the market. Here Mr. Reagan is at his most unpersuasive and implausible.

Mr. Carter's most important -- and most courageous -- contribution has been to begin decontrolling oil prices. A little over half of the country's oil production has now been freed from controls. The high price is enforcing conservation rapidly and effectively. An oil windfall tax is returning part of that price rise to the public -- although not as much as the incessant whining of the oil industry might lead you to think. The federal government is slowly pushing industry and utilities toward other fuels. It's supporting development of the technology to use coal without creating poisonous smoke. The country is embarked on a course that will serve it well -- in the year 2000.

But what if there's a major disruption of the world oil supply next year? There the Carter record is less admirable. To mollify the Saudis, Mr. Carter ceased filling the strategic oil reserve last year and has begun again only because Congress forced him. The country ought to be cutting oil imports much faster, by taxing gasoline heavily. Mr. Carter proposed a gasoline tax in 1977 but, when Congress promptly killed it, he let the whole idea drop. The only candidate in this presidential campaign who has forth-rightly and firmly recognized the need for that gasoline tax is John Anderson.

Mr. Reagan airily dismisses all questions about oil with the comment that America has plenty of energy resources. If he thinks that this country can raise its oil production to meet present demand, as he sometimes seems to imply, he is simply wrong. If he's thinking that coal production can be substantially increased, he's right -- but, as motorists discovered in last year's gasoline lines, all the coal in the world won't get your car to the grocery store until it's transformed into liquid. That's technically possible, but private industry declined to do it without federal aid.

Mr. Reagan trusts the market, but the market's workings are blind and sometimes wantonly destructive. Two hugely expensive oil crises in the past decade have demonstrated the market's limitations in foreseeing trouble. What if there were a sudden drop in oil shipments from the Persian Gulf? Mr. Reagan does not recognize the government's duty to provide shock absorbers against too harsh an impact on the market, and too rapid change.

If Mr. Reagan is elected, he will rapidly discover that the bromides of this campaign aren't enough. What will he do then? He hasn't given voters much indication that he has thought about it.

If Mr. Carter is reelected, there's no indication of any great change in his energy policy. His judgment has been pretty good -- but there is no subject in which that judgment has been more often nullified by his inability to show Congress and the country what he wants to do, why it's right and why they ought to support him actively. His inability to persuade has had especially serious consequences in this area. All the old ideological and regional quarrels over oil and gas persist, slowing national reactions dangerously. Educating the country to the realities of oil and energy is being left largely to events -- to last year's gasoline lines and to this year's war.