President Reagan warned today that congressional rejection of his proposals for the MX intercontinental ballistic missile would be "a blow to our national security that no foreign power would ever have been able to accomplish."

Beginning a week of personal lobbying before the vote on deployment of the MX and development of a new, smaller missile, Reagan said he believes "with every fiber of my being" that both nuclear weapons are "essential to ensuring arms control progress and our nation's future safety and security."

Reagan again endorsed the recommendations of the President's Commission on Strategic Forces, which called for basing the MX in hardened, existing Minuteman missile silos, developing a smaller single-warhead missile dubbed "Midgetman" for deployment in the 1990s, and continuing the pursuit of arms control talks with the Soviets.

The president has previously embraced these goals, but administration officials say they believe that an all-out personal lobbying effort by Reagan will be required to win congressional approval of the commission's recommendations.

Presidential spokesman Larry Speakes said today that Reagan will spend "a good deal of time with congressmen on the MX this week," including "a fair amount of personal lobbying."

Reagan's comments today came on the eve of a scheduled meeting with bipartisan congressional leaders at the White House Tuesday afternoon on the MX and arms control.

The president has attempted to reassure congressional skeptics of his commitment to the arms control proposals in the MX commission report.

He is expected to promise at the meeting with congressional leaders that he will integrate those recommendations into current U.S. negotiating positions at the arms talks with the Soviets. Reagan also is prepared to give congressional leaders written assurances of his commitment to arms control, administration officials said.

"Only when the Soviets are convinced that we mean business will arms control agreements become a reality," Reagan said in remarks at a memorial dinner at Ashland College here for the late conservative Rep. John M. Ashbrook (R-Ohio). "We are not building missiles to fight a war, we are building missiles to preserve the peace."

"The debate over a new ICBM and our entire strategic triad has cost the country millions of man-hours and billions of dollars and it still hasn't been decided," Reagan said.

"During the past 10 years, the Soviet Union has improved, developed and deployed more than a dozen large, new ICBM systems, while the United States has been thinking one, much smaller, ICBM," he said. He noted that the MX commission report was endorsed by 18 senior officials from the three previous administrations, including six former secretaries of state and defense.

In his speech, Reagan also reached back to the roots of his conservatism for a political sermon in which he said that "the central political error of our time" was the expansion of government into "the primary vehicle of social change."

His address was largely directed at conservatives of the "Old Right" wing of the GOP, alongside whom he waged a long and often losing battle against big government in earlier years. Reagan eulogized Ashbrook, who died last year during his Senate campaign, as a symbol of the movement that carried him to the White House.

And, on the last leg of a political swing that included appearances before Hispanics, gun owners and the elderly, Reagan tonight launched a broadside against the Great Society social programs of the 1960s and 1970s.

"The great spending programs failed for the vast majority of poor Americans," he said. "They remain trapped in economic conditions no better than those of a decade and a half ago."