President Reagan, slumping in the polls and under increasing attack in Congress, will try to regain the political initiative tonight with the first prime-time news conference of his administration.

"This is an effort to let the president go unfiltered to the people and not be filtered through the 6 o'clock news," said a senior White House official in describing the event.

The 8 p.m. news conference is part of a new strategy to unleash the "great communicator," as Reagan's aides like to call him, in an attempt to let the president directly make the case for his embattled foreign and domestic policies. Part of this effort will be a 10-week series of live weekend radio reports that will begin this Saturday.

White House officials said the president plans to begin his news conference tonight with a statement calling for negotiations with the Soviet Union to reduce nuclear arms. The proposal is expected to be along the lines of a resolution by Sens. John W. Warner (R-Va.) and Henry M. Jackson (D-Wash.) that calls for the United States to propose to the Soviets "a long-term mutual and verifiable nuclear forces treaty at equal and sharply reduced levels of forces."

"There may be a word or two in our resolution which the president would prefer was not there," Warner said yesterday after he and Jackson conferred with Reagan at the White House. "The basic thrust is acceptable." But White House aides see the president's news conference as more than an opportunity to head off growing public and congressional pressure for a nuclear arms freeze.

The White House view is that Reagan is the best possible administration spokesman to counteract the impression that the president is slipping and that his programs are in trouble.

In resorting to a televised press conference, a format favored by past presidents who found themselves in political difficulty, Reagan will be challenging the conventional wisdom of the Washington press corps, which after several past news conferences has challenged his facts, his statistics and his mastery of complex issues.

"We think the public has a much more favorable view of what the president says in these news conferences than the press does," one White House aide said yesterday.

National and statewide polls almost unanimously show that Reagan's job approval rating has dropped by 20 points or more; the latest California poll shows that he has dropped 27 points in his home state in the past year, from 64 percent to 37 percent.

Reagan often resorts to television when he is in trouble. He built public support for his welfare and tax bills as governor of California with prime-time television speeches.

He did the same as president in 1981 with speeches from the Oval Office in behalf of his economic program. He used television as a major political and fund-raising weapon in two presidential campaigns.

White House aides say that Reagan would have used the more risky format of a prime-time press conference last year, but he didn't have to because the networks honored every White House request for time for a television speech from the Oval Office.

Reagan did not ask for time for a similar speech now, officials said, because the negotiations over his economic program are at a delicate stage.

But he did want to go on television, and last week his trio of top advisers--chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver and counselor Edwin Meese III--proposed the prime-time news conference instead.

Briefing teams coordinated by communications director David R. Gergen are preparing Reagan for the event through a series of discussions and rehearsed questions. Reagan prefers this sort of preparation to spending his time with briefing books.

In addition to Baker, Meese and Deaver, presidential assistants Richard G. Darman and Gergen, budget director David A. Stockman and national security adviser William P. Clark were said to be heavily involved in the preparation.

Gergen announced yesterday that Reagan will begin his series of five-minute radio broadcasts Saturday with a report on the economy.

The broadcasts are scheduled to include two while Reagan is abroad--one from Barbados on April 10 and one from France when the president visits Europe in June.

Yesterday was the anniversary of the attempt on Reagan's life outside the Washington Hilton.

Responding to a question about the shooting, the president said the event "seems a lot longer" than a year ago and added: "I feel fine."