KNOWING THEY'D lose if it came to a vote, Republicans have done as threatened and blocked the Senate from proceeding with campaign finance reform. They say the sticking point is the public financing the bill would provide for Senate campaigns -- that the Democratic bill is a grabby effort by a lazy majority to perpetuate itself in office at public expense; that the present system is not the cozy trade of clout for cash the critics portray; that public finance will create a Senate not more responsive to the public will but less so.

This is a false issue. A comparable public financing scheme has been in effect at the presidential level for three elections now. The view almost everywhere is that it has helped in mucking out the stables. Ronald Reagan has taken public money three times, more than $90 million in all; he seems to have survived it. Bob Dole has indicated he will take it in his presidential campaign. Why at one level is it a cleansing influence, at the other to be deplored? Which side of this no doubt moral issue is the minority leader on?

The current congressional financing system is at the very edge of rot. Sensible members of both parties understand that. The cost of office leaps ahead in every election cycle, more than doubling in 10 years: $3 million for the average Senate seat, more if the seat is contested; $300,000 for a seat in the House. The democratic process drowns in amounts like these; the members have been driven to the PACs, which are only too happy to oblige. The answer is spending limits, but the Supreme Court has said that spending limits are only constitutional if they are part of a quid pro quo. That is why public spending is in this bill. You don't have to take the money, but if you do, you have to abide by the terms on which it is given. If not public spending, what do the Republicans propose? They would bid the price of office to the moon.

Some Republicans have suggested, instead of caps on spending, shifts in the sources and mix of funds. The idea is to let the PACs give less, individuals and parties more. A good beginning -- the individual limit of $1,000 per election has not been changed since 1974 and is much eroded by inflation -- but not enough; these alternatives skate on the surface of the problem. A few others in both parties say the answer is not legislation but a constitutional amendment that would permit spending limits without the window-dressing of public finance. But constitutional amendments take forever, and we wince at the idea of making Swiss cheese of the First Amendment.

The issue is not public-versus-private financing, as the Republicans would have it. The real-world issue is whether and how to limit the role of megadollars in our politics. Yesterday's vote showed that the Democratic bill can pass. The Republicans don't like it, but neither can they want the present system hung around their necks. If not the Democratic bill, then an alternative that will also limit spending and take the Senate off the present race course: that is what the Republicans must help provide.