President Reagan launched the 1982 political campaign today with a three-state midwestern foray aimed at selling his "New Federalism" program and defusing the growing opposition to administration economic policies.

Reagan began his two-day tour with a vigorous defense of his unbalanced budget at a fund-raising reception for Sen. David F. Durenberger (R-Minn.), who is seeking reelection to a second term.

His campaign kickoff got a mixed reception, however. The presidential motorcade was greeted by several hundred demonstrators who waved anti-administration placards.

One read, "Welcome President Hoover." Another proclaimed, "Stick it to Stockman," a reference to the president's controversial budget director.

In his speech here to about 1,500 Republican contributors, the president acknowledged that unemployment had increased and was "the cruelest thing that can happen to people . . . who want to work and can't find work."

But he staunchly defended his economic policies, saying they had already reduced inflation and interest rates and would spur a strong economic recovery if given a chance.

Reagan denounced as "demogoguery" Democratic calls for a reduction in the defense budget or a postponement of the tax cuts enacted last year. The president said the tax cuts are necessary for economic revival and that defense boosts will be needed until the mid-1980s in order to match previous increases in the Soviet defense budget.

The purpose of this political trip, according to an aide, is threefold. First, the president hopes to generate some enthusiasm for his federalism plan in speeches to the Iowa and Indiana legislatures on Tuesday.

Second, he intends to meet with local newspaper and television executives and editors in Minneapolis, Des Moines and Indianapolis in a series of private meetings and interviews that one adviser said will "avoid the filter of the national press."

In his speech tonight, Reagan said: "You have to get at least 50 miles away from the Potomac to get back to the real world."

Third, Reagan wants to demonstrate, as he did here in pep talks to a $500-a-plate dinner and a $15-a-head reception, that he does not intend to abandon Republican candidates, many of whom face difficult reelection battles in 1982 because of Reaganomics.

Durenberger, however, does not appear to be one of those in trouble. Republican polls show him an overwhelming choice for reelection, and the president's early appearance here was mostly for fund raising. Similar trips and speeches are planned in the near future in behalf of supposedly safe incumbent Republican senators in New Mexico, Wyoming and Utah.

Reagan's defense of the budget on this trip is designed to show, as one aide put it, that "we're not trying to hide behind federalism." The view of White House senior advisers is that Reagan's call for additional budget cuts is more popular in the country than it is in Washington and that Americans will respond to Reagan's personal appeal for his programs.

Even so, the White House political positioning for 1982 is essentially defensive. The yardstick that the administration will try to get the press and public to accept is the "normal, off-year loss" that the administrations in power usually suffer in mid-term elections. Using this guide, the administration will try to represent anything less than a loss of 36 congressional seats as a victory.

This "pre-positioning" was one of the themes discussed by Reagan's advisers at a secluded meeting at Camp David last Friday. The mood of this meeting, according to participants, was pessimistic but not grim. Several advisers, as well as Reagan pollster Richard B. Wirthlin, believe there is still significant support for Reagan among American voters, most of whom are viewed by the White House as still wanting Reagan to succeeed.

These early political forays by the president in defense of his policies are intended to show the flag to Republicans who are becoming increasingly discouraged by mounting unemployment and concern over renewed high interest rates.

"It's very early in the year and no one is throwing in the towel," said one White House adviser. "We all recognize, however, that the economy has to improve markedly or it won't matter who makes speeches or where they make them."