President Reagan, who returned to Washington this evening, plans this week to increase his high-pressure salesmanship for proposed defense budget increases and to begin building support in Congress for deploying the MX missile.

The president is known to be considering a major speech on the MX within two weeks, and his special MX commission is expected to report to him within about 10 days its final recommendations for basing the nuclear missile.

Reagan suffered a defeat on the MX last December when Congress rejected his plan to base the missile in the so-called "Dense Pack" formation, and he is now calling for bipartisan support to achieve a basing mode.

In addition, the president's aides say he plans soon to offer evidence of Soviet use of chemical warfare in violation of international treaties.

He would do so, they said, to keep pressure on the Soviets and supporters of a nuclear weapons freeze by reiterating that the Soviets have proven untrustworthy on treaties far less demanding than one involving an all-out nuclear weapons freeze.

Meanwhile, aides said, the administration is considering offering lower defense spending figures to Congress as a compromise, although no such numbers have been put in final form.

"It is the subject of internal discussions now as to how it is going to be handled," a senior White House official said. "But we should have a proposal ready in the next few days."

Reagan also faces resumption this week of his fight in Congress against the nuclear freeze movement. A vote on a freeze proposal is expected soon in the House, and, after sizable Easter weekend demonstrations in western Europe, the president is seeking to avoid losing support at home for his opposition to a freeze.

The administration's campaign against the freeze movement started last week with a series of presidential actions intended to diminish criticism of him as an unbending militarist not interested in negotiating an arms reduction treaty with the Soviets.

After a speech in which he proposed creating a defense against nuclear missiles to end escalating missile stockpiling, Reagan offered the Soviets a new treaty proposal Wednesday. This softer Reagan also said that the impact of a nuclear freeze could endanger chances for Soviet agreement on reducing weapons.

"We Americans are an impatient people," the president told the Los Angeles World Affairs Council Thursday. "Often this is a source of strength. In a negotiation, however, impatience can be a real handicap . . . . If one side seems too eager or desperate, the other side has no reason to offer a compromise and every reason to hold back, expect that the other more eager side will cave in first."

Reagan's speech indicated his belief that he is bargaining fairly and reasonably. He said his proposals to the Soviets had not been made on a "take-it-or-leave-it basis."

Similarly, he offered proposals in ongoing negotiations that he said were meant to end all nations' preparations for war. "Never before in history has a nation engaged in so many major simultaneous efforts to reduce the instruments of war," he said.

In that context, Reagan added that he soon would provide evidence of alleged Soviet use of chemical warfare. He said the Soviets had shown disregard for existing arms control treaties and were using toxic weapons in Afghanistan, Laos and Cambodia.

Despite the European demonstrations, administration officials said today they felt that the president's recent speech opposing the freeze and presenting new treaty proposals were being received favorably by more Americans and Europeans.

"Response among European leaders has been enthusiastic," White House communications director David R. Gergen said. "And although it is early, response . . . in the United States seems good. Of course, some people will continue to oppose deployment no matter what the Allies or the president may do."

Administration officials said no plans were being made for any arms treaty offer to the Soviets. Soviet Foreign Minister Andrei A. Gromyko said Saturday that Reagan's recent arms treaty offer was "unacceptable."

Administration officials said there is little optimism for agreement soon with the Soviets. Consequently, the administration's focus remains on blunting the effect of the nuclear-freeze movement and offsetting attempts to lower the amount of the proposed increase in defense spending.

On their vacation here, President and Mrs. Reagan enjoyed blue skies and 80-degree temperatures. The president went horseback riding and was visited by his children.

This morning, the Reagans made an unannounced trip to attend Easter services at Santa Ynez Valley Presbyterian Church. Because of the visit's element of surprise, the Secret Service dropped the customary weapons search of congregants.