From the grass roots to prime time. The public demand for stopping the nuclear arms race has grown so much that President Reagan made arms control the centerpiece of his first evening news conference. This demand had already brought the introduction of two varying congressional resolutions for a nuclear freeze. Now it will be up to the public whether the momentum against nuclear weapons is sustained - and, if so, which congressional approach is to prevail while the administration follows up on the President's plea for peace and dramatic arms reductions.
Immediate benefits of Mr. Reagan's heartfelt response to a yearning public are evident:
* It helps Western European leaders persuade their constituencies that the American President is not a trigger-happy cowboy ready to accept ''limited'' nuclear war on their soil.
* It prepares the way for Mr. Reagan's opportunity to convince Europeans in person when he visits their continent in June - a trip wisely scheduled despite predictions of protest demonstrations that the White House must have been tempted to avoid.
* It confirms that American progress toward negotiating long-term strategic arms controls is getting back on track after being diverted by the Poland situation. Mr. Reagan properly stressed the importance of careful preparation for the expected arms talks - no doubt well aware that the Carter administration was stung when it, too, began by seeking US-Soviet reductions rather than mere limits.
* It most particularly lets the American public know that the ear of their President can be gotten when individual citizens come together and speak out. Mr. Reagan said he shared ''the determination of today's young people'' that war must never happen again -- a tragedy that would be ''rendered even more terrible by the monstrous inhumane weapons in the world's nuclear arsenals.'' This is a determination shared not only with young people but with citizens of all ages, notably the parents who do not want to leave a tragic legacy to their children.
Mr. Reagan timed his March 31 news conference to reach as many of them as possible. In effect he challenged them to keep up the struggle against nuclear weapons with him on their side. And he challenged them and their representatives to choose between the two congressional steps being offered at right now.
One of these bipartisan resolutions, introduced by Senators Hatfield and Kennedy, proposes a worldwide, verifiable nuclear freeze, with weapons reductions to be negotiated later. Polls last month found a majority of Americans supporting an immediate freeze between the Soviet Union and the United States.
The other resolution, introduced by Senators Warner and Jackson, proposes a freeze only after reductions on a basis of US-Soviet equality. Mr. Reagan said this one was consistent with his policies. In the current controversy over whether there is rough nuclear parity between the US and the Soviet Union, the President sided with those who find the Russians, on balance, to be superior. Thus this second freeze proposal would require the US to build up to equality before both sides undertook the reductions that would permit a freeze.
Among the questions for Americans is whether the Russians would insist on matching a continued US buildup, thus prolonging the arms race. Whether the Russians would still have economic or other incentives to negotiate arms reductions if the situation were frozen today. Whether sufficient verification could be agreed upon. Whether, finally, enough is enough now or will be later.