Sen. Max Baucus (D-Mont.) was in Helena last weekend for a state Democratic Party reception and dinner. The subject most people wanted to talk about, Baucus said afterward, was neither the state of the economy nor the Democrats' prospects for November, but nuclear war.

"I was astounded," Baucus said. "By five or six to one, the people who came up to me wanted to talk about nuclear war and arms control."

As recently as January there was hardly a word to be heard in this city about the issue of nuclear arms control. Yet today a freeze on nuclear weapons has become one of the capital's hottest topics. Baucus is one of many members of Congress from both parties who are feeling the warmth from what Rep. Timothy E. Wirth (D-Colo.) calls "the most powerful, spontaneous grass-roots movement I have seen since I was elected to Congress" in 1974.

The sudden, unexpected pressure from the hinterlands to do something about nuclear arms has set off frenzied activity in Washington. Tonight President Reagan holds his first prime-time news conference to announce a new proposal for arms control negotiations. On the Hill, members of both houses are circulating competing resolutions calling for different kinds of freezes or controls on nuclear weapons. Yesterday the House staged an unusual marathon debate on the issue to give members a chance to go on record.

The degree of public concern behind these Washington maneuverings is "amazing," according to Rep. John F. Seiberling (D-Ohio).

But just where this new phenomenon came from is the subject of some debate. Many members of Congress and aides to members said in interviews that provocative Reagan administration talk about such subjects as limited nuclear war has contributed significantly to new popular misgivings. Other factors cited as possible causes are the anti-nuclear movement in Europe, the debate over high military spending at a time of budget austerity for domestic programs, and the psychological consequences of the collapse of the Strategic Arms Limitation Talks that had continued from 1969 until 1979.

The political pressure on Washington to do something about nuclear weapons has materialized so quickly and unexpectedly that even the leaders of grass-roots groups around the country are surprised. "Our initial timetable," said Randy Kehler, a leader of the national movement to freeze Soviet and American nuclear arms, "based on guesswork, was for introducing a congressional resolution later this year" after building up organizations in individual congressional districts.

But, Kehler said in a telephone interview from his office in St. Louis, "we realized that the freeze is an idea over which the freeze campaign can have no monopoly."

In fact, since the beginning of this year numerous resolutions calling for a freeze or something like it have been circulated in Congress, and two have now attracted widespread support in both houses. Proponents of both acknowledge that they are riding a wave of popular sentiment that originated not in Washington, but out in the country.

The White House yesterday embraced one of these resolutions, introduced in the Senate by Henry M. Jackson (D-Wash.) and John W. Warner (R-Va.) and co-sponsored by 57 senators. Though it uses the word freeze, the Jackson-Warner resolution actually says that the Soviets now have greater nuclear power than the United States, and that a freeze on weapons should be imposed after the superpowers negotiate "equal and sharply reduced levels of forces."

It is a measure of the intensity of feeling on this issue that proponents of an immediate halt to the arms race and negotiations to establish a mutual Soviet-American freeze sharply attacked the Jackson-Warner proposal as soon as it appeared.

Sens. Edward M. Kennedy (D-Mass.) and Mark O. Hatfield (R-Ore.), principal sponsors of a pro-freeze resolution that has 24 Senate supporters, issued a statement yesterday calling the Jackson-Warner proposal "a blank check for the Reagan administration to continue the nuclear arms race. We also regard the resolution as dangerously deceptive, because it pays lip service to the concept of a freeze while actually pushing the nation into yet another spiral of the arms race."

If the origin of this new interest in nuclear freezes is any guide, however, the issue is not whose Senate resolution will get the most co-sponsors or votes. As perceived by many in Congress, the freeze phenomenon is a political event coming from outside Washington, which Washington may find difficult to control.

"What we're engaged in right now," said an aide to Kennedy, "is catching up with the country."

Sen. Daniel Patrick Moynihan (D-N.Y.) said he thought President Reagan should take the leadership of the freeze movement himself. If he fails to do so, Moynihan said, the present popular pressure for a bilateral, negotiated freeze on nuclear weapons could turn into "a strong, unilateral disarmament movement." Moynihan, traditionally a hawk, has co-sponsored the Kennedy-Hatfield resolution.

Many others in Congress said the White House had erred by not trying earlier to co-opt the freeze movement. Randall Forsberg of Cambridge, Mass., a leader of the national campaign who is credited with coining the "freeze" idea, said the Reagan administration "probably made a mistake by opposing this movement rather than embracing it."

In fact, although the first Reagan administration reaction to the freeze idea was sharply hostile, it has lately been more moderate, and yesterday the White House endorsed the Jackson-Warner version of a freeze resolution, which uses the terminology if not the substance of the freeze campaign's argument.

Campaign organizers outside Washington hope to influence the outcome of the 1982 and 1984 elections. "Congress is most sensitive at election times," Kehler said, noting that local chapters of the freeze campaign have been organized by congressional districts. Some kind of organization now exists in 43 of the 50 states and 279 of the 435 House districts, he said.

Campaign organizers insist that they have barely begun to mobilize public opinion, a thought that concerns many members of Congress. "We have nowhere near reached the full potential of this movement," Kehler said. "It's sort of doubling every couple of months," added Forsberg.

Rep. Edward J. Markey (D-Mass.), whose personal lobbying has helped win 160 co-sponsors for a House resolution identical to Kennedy-Hatfield, predicted flatly that the economy and nuclear arms control will be the two major issues in 1982's congressional elections.

Supporters of the freeze idea generally agree that its ultimate purpose is to stop the arms race, but they are not unanimous on just how. Many backers hope to pressure the government to pursue a freeze, while others think the main objective should be to convince the Reagan administration (or its successor) that the country rejects a business-as-usual approach to nuclear weapons and demands action to reduce the danger of nuclear war.

The freeze issue has split the community of arms control experts. Many have refused to endorse the freeze because of specific qualms about the feasibility of achieving it. Others have said the political benefit of a mobilized public more than compensates for specific technical problems surrounding a freeze. Freeze organizers are openly gleeful that they have upset experts, whom they blame for letting the arms race continue for so long.