Once upon a time, it was Jimmy Carter and those big-spending Democratic congresses of the last several decades who were responsible for high unemployment and economic upheaval in the United States.

Now, with double-digit unemployment a reality, President Reagan has discovered that "structural" problems are to blame for the nation's economic woes.

Suddenly, he sees the necessity of engaging in the kind of job retraining and relocation programs that economists and politicians of both parties have been talking about for some time.

This new Reagan discovery, tried out on the road in Chicago last week, will be a centerpiece of the president's State of the Union message Tuesday night. Reagan will acknowledge that the decline of smokestack industries such as steel and auto represents a basic economic fact of life in the computer age and use this recognition to deflect criticism of his economic policies.

The new focus will be reinforced Wednesday when the president flies to Boston for drop-ins at a retraining center and a high-technology electronics factory in a poor neighborhood.

"We are suffering the structural problems of an industrial society being transformed into more of a service and information society . . . ," Reagan said last week in Chicago.

It's a big change for a man who once blamed much of what was happening on the "failed leadership" of President Carter.

Watchword for the president's new approach to the Democrats, as codified by longtime Reagan adviser Lyn Nofziger: "We are all bipartisanites together."

And then there's the White House press room crack about what Reagan really will be doing when he goes to Boston: "Giving a lecture on his new economic theories at Harvard."

Behind the new "public diplomacy" policy that trickled out of the administration last week lies another unpublicized dispute between key members of the happy-talk quartet at the White House. The disputants in this case are chief of staff James A. Baker III and national security adviser William P. Clark, who have different views of what the president ought to do in combating the anti-nuclear arms movement in the United States and Europe.

Clark argues that the political decision made at Baker's urging last November to keep Reagan and the White House away from the nuclear-freeze issue was a mistake.

Nuclear-freeze proposals carried eight states and the District of Columbia, losing only in Arizona. But the margins of victory were less than expected, particularly in California. Clark believes that the outcome would have been different if the president and his spokesmen had become forcefully involved. Baker sees enormous negative political potential for the administration in the nuclear-freeze issue. He is convinced that Reagan would have damaged himself without defeating the nuclear-freeze initiatives if he had become involved last November and believes it was unwise of the president to talk about purported communist involvement in the peace movement.

Instead of a hard sell on the Soviet menace, Baker prefers that Reagan stress the need of an arms control agreement and reconciliation with the Soviet Union.

Meanwhile, the Soviets are picking up the Reagan line. Arms negotiator Edward Rowny, briefing reporters after his meeting with the president Friday, said his Soviet counterparts admire Reagan because he's a man who "stays the course."

Also staying the course, in his way, is Rep. Jack Kemp (R-N.Y.), still hanging in there for supply-side economics even if Reagan is not. At a dinner party last week, the former Buffalo Bills quarterback was introduced as the next president and quipped in response: "No, I want a position of real power: commissioner of the National Football League."

One of the first leaks from the Cabinet meeting last Tuesday was the stirring news that the president had made yet another speech denouncing leaks. "Teamwork" and "unity" were the bywords of the message as the administration girds for a tough battle on the budget.

"You're part of the team and don't volunteer answers was the basic message," one participant in the meeting said. "Keep your mouth shut and fight for the package, and don't send up subordinates to the Hill who will depart from the premise or say too much."

Timely Lapse of the Week: Chief of staff Baker, discussing how Reagan had kept his promises during his first two years, had an uncharacteristic lapse. In a speech, Baker couldn't recall one of Reagan's pledges. Then it came to him that the missing promise was "balancing the budget."