Sen. Henry M. Jackson (D-Wash.), widely rumored as a possibility for secretary of defense or state in a Reagan administration, said yesterday that how the United States manages its foreign policy in the decade ahead will be the crucial factor in the daily lives of Americans.

Speaking at a seminar on Capitol Hill arranged by Sen. John Heinz (R-Pa.), Jackson drew on the two areas that are his congressional specialties -- energy and defense -- to lay out what he sees as the fundamental problems facing the western nations for the balance of this century.

Those problems, Jackson said, will be a continued dependence on sometimes unreliable foreign suppliers of oil, and the decline of real and preceived military power on the part of the United States in comparison to the Soviet Union.

These two trends have been going against the United States, he said, and they have an enormous impact on everything from interest rates to the bond market and the entire domestic economy. "So the management of foreign policy becomes the bottom line, and it's that management that will determine the availability, the reliability, the amount and the price of oil," he said.

As for U.S. defenses, Jackson said the task that lies ahead is rebuilding strength for the long haul -- not in an on-again, off-again fashion. "We have to understand that this is a protracted, adversarial type of conflict" with Moscow, he said. "The Soviets are not wild. They do not move into areas where the West is strong and united."

The United States cannot shout at the Russians, he said, advocating instead the Theodore Roosevelt approach of speaking softly and carrying a big stick.

Reagan named Jackson to an interim advisory board on foreign policy during the transition period. But sources close to the senator said that no official Reagan staff member has contacted Jackson about a possible cabinet job. Jackson is said to be undecided if one were offered.

Among his supporters, Jackson is most often viewed as a likely defense secretary. But others close to him view him also as secretary of state material because of his views on U.S.-Soviet relations and on the linkage between policy and power. These views were the ones that seemed to be on display in yesterday's remarks.

It was revealed yesterday that Richard Perle, a former key aide to Jackson on defense matters for many years, will be part of the Reagan transition team working in the Pentagon. Aides to the senator, however, said this does not mean that Jackson might he heading for the Pentagon with him.

Another potential appointee to a high Reagan administration post, retired Gen. Alexander Haig, also appeared before the Heinz-sponsored seminar.

Haig also stressed the growth of Soviet military power and said it is vitally important that the United States restore "an umbrella of confidence" for its allies. He argued that American strength must be bolstered quickly so that the United States will be in position to cope with a potentially more activist leadership in the Kremlin.

Haig deplored what he called the "excess welfare state" in the United States, arguing that supposed beneficiaries of welfare are becoming the victims of inflation and unemployment and that this erodes allied confidence in this country. Haig's emphasis on allied perceptions also underlines what many view as a major quality of his that could be attractive to Reagan, namely his experience in dealing with allied leaders during four years as NATO's top commander in Europe.