President Reagan's START representatives are going to Geneva with a brisk political tail wind. As chief negotiator Edward Rowny has said, they cannot ignore public opinion the way their totalitarian Soviet counterparts can. All the more important then that Americans heighten the pressure against the nuclear arms race to which their leaders and politicians are already beginning to respond. The intent, of course, would not be to push US negotiators into giving away the store to Moscow - as if a Reagan negotiator could be imagined doing any such thing - but to keep a momentum toward arms reductions of mutual and indeed worldwide benefit.
Thus the grass-roots movement for a mutual and verifiable nuclear arms freeze provides a valuable political nudge to Mr. Reagan even though he seeks to serve the citizenry's hope through other means. He has to reckon the political cost of failing to achieve progress by an alternative route while leaving the freeze to the Democrats.
Some Republicans have evidently already seen hazards in letting the Democrats monopolize the issue. Last week seven Republicans, ranging from Jim Leach in Iowa to Millicent Fenwick in New Jersey, joined with Democrats on the House Foreign Relations Committee to pass a freeze resolution.
But the Senate Foreign Relations Committee, with its Republican majority, had earlier rejected a freeze resolution on a party line vote. And during this past weekend's Democratic midterm conference there was plenty of disposition to make partisan hay of the issue.
Not that all Democrats favor the freeze on nuclear weapons at present levels. Senator Jackson is a notable exception (though he sponsored a measure endorsed by a Senate majority for a future halt at ''equal and sharply reduced levels'' that would not prevent the current US arms buildup). But many Democrats see political capital in the freeze. If they turn out to be right in the voters' eyes, the present tail wind for START could become a gale.
President Reagan is too shrewd a politician not to be aware of the challenge. What he has to address is the simple conviction of the freezers that both the US and USSR have so many excess deterrent arms that there is no reason to produce more while reductions are being sought. Many have accepted his argument that the freeze would ratify a dangerous imbalance and, in effect, be an unacceptable delay and diversion in the effort to obtain mutual reductions.
Here is where Mr. Reagan challenges himself to sustain the impetus against delay during the negotiations which he has so aptly labeled START -- Strategic Arms Reduction Talks. To move from arms limitation to arms reduction is a stride forward in the attempt to curb the threat of massive nuclear arsenals. To maintain the mutual deterrent at lower levels has to be inviting not only to the United States, with its huge arms-generated deficits, but to Moscow, with its proportionately even larger arms expenditures in a far worse economy.
The US has staked out a strong opening position on strict ''equality.''
The USSR wants to combine equality with something called ''equal security,'' which may mean taking account of the various fronts it has to cover, with China an adversary as well as America. Cannot there be some meeting of the minds here if, as negotiator Rowny says, the US aim is not superiority but deterrence?
At least let tomorrow's start of START foreshadow the start of reducing differences, with President Reagan letting his men know he expects results.