The Reagan administration launched an intensive intercontinental lobbying effort this week to prevent the MX missile from being blown up on the pad by the House Appropriations Committee today in an important test of President Reagan's defense buildup.

Reagan telephoned from Brazil on his tour of Latin America to urge several committee members to vote against an amendment sponsored by Rep. Joseph P. Addabbo (D-N.Y.) to stop the MX from going into production.

Vice President Bush made similar calls here, while Defense Secretary Caspar W. Weinberger phoned from Brussels, where he is attending a NATO alliance defense ministers meeting, to argue that losing the MX vote would be sending the wrong signal to the new leadership in the Kremlin.

Pentagon research director Richard D. DeLauer and Air Force generals at the Strategic Air Command in Omaha, Neb., also were enlisted in the campaign to save the president's plan to put 100 MX intercontinental missiles, each carrying 10 nuclear warheads, in a closely spaced "Dense Pack" formation in Wyoming.

Addabbo said last night that the Reagan administration's lobbying effort had become so intense that he no longer felt confident he would prevail in cutting $988 million for production of MX from the fiscal 1983 defense appropriations bill now before the committee. He said that the odds had shifted, declaring that he was going into the committee session with 24 votes against 27 for Reagan, with the rest undecided.

In a letter to his House colleagues arguing that they would be wasting "well over $35 billion" by allowing the Pentagon to go ahead with the missile, Addabbo wrote, "To hear the president talk about MX, you'd think the future of Western civilization hung in the balance if we did not proceed."

Lobbyists on both sides said the vote in the House committee comprised of 33 Democrats and 22 Republicans was too close to call.

One of the undecided committee members was Republican Rep. Virginia Smith from central Nebraska's sparsely settled Third District, who found herself inundated by administration pleas on behalf of MX, beginning with a telephone call from President Reagan early Sunday morning.

"Hello Virginia," she quoted the president as saying. "I'm sure that you are glad that the MX is not going into your district."

"His key point," Smith said last night, "was that unless we support the MX initiative we will be sending the wrong signal to the Soviets. It wasn't a hard sell, but he ended up by saying, 'I hope I can count on your support.' "

She said Air Force Gen. George Miller, deputy commander of the Strategic Air Command in Omaha and an old aquaintance of hers, followed up the president's call with one of his own. "SAC did not buy Dense Pack wholeheartedly, he said," according to Smith's account, "but now feels it is viable and is the best way to go."

Smith said she expressed concern to Miller that anti-ballistic missiles might be deployed near her district to protect the Dense Pack formation of MX missiles just across the Nebraska border outside Warren Air Force Base near Cheyenne, Wyo. "He said it was extremely unlikely that this would happen, certainly not in the near future."

She also was called by DeLauer from the Pentagon, who offered to come to her office to brief her on the virtues of Dense Pack. "I don't think I can say anything more persuasive than the president has said," DeLauer told her in what Smith described as a low-pressure pitch.

Bush topped off the administration's lobbying of Smith with a phone call yesterday. She recalled that he said: "I'm not going to use the term bargaining chip for the MX, but it is an important aid to the arms control negotiations."

The Air Force also tried to counter critics of the MX and the Dense Pack deployment plan by issuing a statement saying a Nov. 8 test of how well the hardened MX silos would hold up under Soviet attack "surpassed Air Force expectations in terms of resistance to blast effects."

Col. Mike Terrill, who runs the Air Force's MX information office from the top floor of the Pentagon, said the test showed each MX missile could be wrapped in enough concrete and steel in the Dense Pack formation to survive attack by a 25-megaton Soviet warhead.

Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee, Chairman Ted Stevens (R-Alaska) of the Senate Appropriations subcommittee on defense and Sens. Henry M. Jackson (D-Wash.) and John W. Warner (R-Va.) also joined the lobbying act by signing a letter linking MX to success at the negotiating table with the Soviets in Geneva.

It urged congressional colleagues to "avoid making a hasty, ill-informed decision" such as denying $988 million in MX production money as Addabbo is advocating.

Both Addabbo and Stevens have pledged to send the fiscal 1983 defense appropriations bill to the floors of their chambers for a vote during the three-week lame-duck session of Congress rather than keep the Pentagon at a low level of funding under a continuing spending resolution for the entire government.

If the bills now at issue are stripped of MX production money, the administration almost surely will try again next year to fund the missile program. However, one administration official said yesterday, "It will be a real kick in the midriff if we lose on the MX tomorrow" in the vote by the House committee.

Asked last night which way she would vote after being subjected to the administration's lobbying blitz, Smith, who says she has more cows than people in her district, replied: "There's been a change in my district. There is now a feeling that we're putting more into defense than we should. People feel there is too much waste. I'm not sure how I will be voting."