The President was right on target. In the opening sentence of his budget message to Congress he said. "The first complete budget of any new administration is its most important. "Why?

Because, he explained, "it is the administration's first full statement of its priorities, policies, and proposals for meeting our national needs." In short, if you want to know what a new administration is all about, examine the budget.

In the last few days, many Americans, having done just that, are finally concluding - in some cases reluctantly - that Jimmy Carter is fundamentally a conservative. In fact, the most conservative Democratic President since Grover Cleveland was elected in 1884.

That, of course, is disappointing to the liberals who did so much to elect him in 1976, but, in all fairness to Carter, they have little reason to complain that he deceived or betrayed them. Like most candidates he tried to please everybody, but, in the final analysis, he made no great effort to conceal his underlying conservatism, especially in fiscal affairs.

In his 1976 acceptance speech he made it clear that - like countless Republican Presidents - he wanted to balance the budget. As for the Humphrey Hawkins full-employment bill, the supreme plank of the party platform, he gave it only grudging, limited approval.

As far back as July 31, 1970, when Carter was running for governor of Georgia, he said, "I was never a liberal, I am and have always been a conservative." His closest associates confirmed it. "He's a very conservative fellow," said Bert Lance, his first budget director. And Hamilton Jordan, the president's top political adviser, also said, "On fiscal matters, Carter is very, very conservative."

Months before the 1976 Democratic convention, I had occasion to note that if Carter reached the White House he would become the first businessman President in the history of the United States.

I must confess, however, that I never imagined a Carter State of the Union speech would prompt a right-winger like Sen. Barry Goldwater (R-Ariz) to say, "I made that speech in 1964, and I was defeated."

Carter's call for a timid $25-billion tax cut was even too conservative for the Republican leader of the House, Rep. John Rhodes of Arizona, who thinks the reduction ought to be twice as big. "If we really wanted to accomplish all the things which I think are necessary for the economy," Rhodes said, "it would take a $51-billion tax cut." Last year, when the President was still emphasizing his determination to balance the budget, despite the high unemployment rate, Rhodes gleefully remarked, "The President is sounding for all the world like the Republican Party platform."

Most of Carter's political initiatives during his first months in office, it is now known, were suggested in a private memorandum written by Patrick Caddell, the President's personal political pollster. "We have an opportunity," Caddell told Carter, "to co-opt many of their [the Republicans] issue positions and take away large chunks of their normal presidential coalition."

"Unfortunately," he added, it is those same actions that are likely to cause rumblings from the left of the Democratic Party." He warned that Carter's real opposition would come from an "antiquated and anachronistic" group of "traditional liberal" and "Young Turk" Democrats.

In summing up his first State of the Union speech, Carter said: "Government cannot solve our problems. It cannot set our goals. It cannot define our vision. Government cannot eliminate poverty, or provide a bountiful economy, or reduce inflation, or save our cities, or cure illiteracy, or provide energy . . . ."

No other Democratic President in this century has ever taken that line. It contradicts everything the party has stood for since the time of Woodrow Wilson.

Without government, there wouldn't be an 8-hour day, or a 40-hour work week, or a minimum wage, or unemployment insurance, to say nothing of Social Security, public health, universal education, Medicare, environmental protection, national security, law enforcement, occupational safety, protection of savings, and countless other indispensable services.

It is astonishing to hear Carter talk like that, since at the same time he is sending messages to Congress calling for government action to solve problems like energy, poverty, urban decay, and a lagging economy, among other things.

Carter recently incautiously said he would judge the effectiveness of his State of the Union message by the reaction of the stock market. As of yesterday, the Dow Jones industrial average had dropped to 764.12, falling 67 points in the first weeks of the new year to hit the lowest level since April 1975. Not a very good testimonial from the country's millions of investors, most of whom are supposed to be conservatives themselves.