In the face of mounting adversity, President Reagan reached back last night to the fundamental vision that propelled him into politics--the belief that the federal government must be cut down to size.

Whether or not that vision ultimately prevails, Reagan, in his first State of the Union address, served notice that he does not intend to suffer the fate of presidential predecessors who allowed events to force them away from their original agendas.

Ronald Reagan, confronted by an economy in trouble and other unforeseen difficulties, is sticking to his basic script.

What Americans heard from their president last night was vintage Reagan. His far-reaching proposal to turn more than 40 programs over to the states was News Analysis News Analysis drawn straight from the controversial "$90 billion" speech that put him in political hot water in the 1976 campaign. His peroration, declaring that America is "the last best hope of man on earth," came from even further back--the October, 1964, address for Barry Goldwater that thrust Reagan onto the political stage as the best living hope of American conservatives.

To those who said Reagan had reduced taxes too much, the president promised further reductions. To those who said the administration already had dumped too many programs on state and local government, Reagan promised more of the same. To those who said that his economic program was a failure, Reagan pledged to ride it out until it brings results.

"If we had not acted as we did, things would be far worse for all Americans than they are today," Reagan said at a time when unemployment is approaching its highest level since World War II. "Inflation, taxes and interest rates would all be higher."

As usual, Reagan's optimism was boundless.

"Don't let anyone tell you that America's best days are behind her --that the American spirit has been vanquished," he said. "We've seen it triumph too often in our lives to stop believing in it now."

And as he did in his inaugural address a year ago, Reagan invoked heroic leaders of the past--Washington, Lincoln, Churchill, Franklin Roosevelt, MacArthur, Eisenhower and John Kennedy--in behalf of his familiar message that the domestic activities of the federal government must be reduced in size and scope at the same time the nation is arming to meet the Soviet threat.

What Reagan did in his speech last night was emerge as the hero of his own drama, triumphing over his own supposedly faint-hearted advisers and assorted Republican congressmen who have urged him in recent weeks to increase taxes. He had been urged to tax cigarettes and liquor, among other things, and to be "realistic" about impending federal deficits that a year ago he was saying would disappear by 1984.

Instead, Reagan did what he wanted to do. He held fast to the fundamental conviction that he can eventually reduce the size of government by starving its revenues. He has always talked this way, but the success of his 1981 tax bill gives him a real weapon for accomplishing his purpose. As long as Reagan can hold out against demands for tax increases, he has a noose around the size of the government in Washington.

There are those close to Reagan who believe that the president's determination reflects his own deeply held philosophical belief and an intimation of mortality that has become stronger since the attempt on his life outside the Washington Hilton last March 30.

Repeatedly, in discussions with his close advisers, Reagan has expressed the view that if he does not succeed in reining in government, it will become impossible for his successors to do. While Reagan has delegated many decisions on many other issues, he has clung to his fundamental conviction about the issue that is most important to him.

The politics of what Reagan is proposing involves high risk, in the sense that he runs the danger of being seen as insensitive to the plight of those who suffer from administration policies and of being an easy target in 1982 congressional elections. But the politics of the alternative--trimming his sails and giving in to his critics--might be even riskier.

The essential debate within the administration on the issue of raising taxes centered on how Reagan would be viewed by Americans if he ran out on his own program, and on what the Democrats would do to any tax-increase measure once the president put one before Congress.

On both counts, Reagan's political intuition was that he stands to gain more by sticking to his policies than by abandoning them.

"If the president gave up on his programs before they had a chance to work, how could anyone else be expected to believe in them?" a longtime adviser said in defending Reagan's reluctance to boost taxes.

Reagan's determination to hold the line spilled over into foreign policy last night, though most of his speech was focused on domestic issues. His rhetoric about the Soviet Union was not as harsh as it used to be, but his basic position was unchanged.

"Our foreign policy must be rooted in realism, not naivete or self-delusion," he said. "A recognition of what the Soviet empire is about is the starting point. Winston Churchill, in negotiating with the Soviets, observed that they respect only strength and resolve in their dealings with other nations."

Strength and resolve is what Reagan displayed last night in refusing to give ground on the battle lines he has drawn with Congress and the nation. While he talked hopefully of continuing cooperation with a legislative branch he described as "a Congress of destiny," Reagan's unwillingness to retreat may have signaled a time not of compromise but of confrontation.

"The only alternative being offered to this economic program is a return to the policies that gave us a trillion-dollar debt, runaway inflation, runaway interest rates and unemployment," Reagan said.

That is the way Reagan views the steady line of social programs that began with the New Deal and culminated in the Great Society. Reagan means to overturn many of these programs. His speech last night was a renewed declaration that he will not be turned easily from his course.