KANSAS' SENATORS are both Republicans. On five cloture votes so far, both have supported the party's filibuster against the campaign finance reform bill on the Senate floor. Now one of them, Nancy Kassebaum, has indicated she will vote to let the Senate proceed. She continues to be opposed to some parts of the bill, will try to change them and reserves the right to vote "no" on final passage. But the obduracy with which her party has greeted this important legislation doesn't suit her. Nor should it.

Mrs. Kassebaum becomes the third Republican to agree to shape the bill, after Robert Stafford of Vermont and John Chafee of Rhode Island. Their example should hearten the other moderates in the party. This is an issue on which these middle-roaders now have the votes to make the difference, to exercise great leverage and precisely the rational leadership to which they so often say they aspire. We will see.

Perhaps Mrs. Kassebaum's constructive example will also move her fellow Kansan, minority leader Robert Dole. That would be the best of all. A leading Republican objection to the bill continues to be that it would use public money to help finance some campaigns. But Mr. Dole is himself about to accept a very large amount of federal matching money to help finance his promising presidential campaign. If the public funds don't tarnish the one, how can they tarnish the other?

The problem this bill seeks to meet is simple bloat. Congressional campaign costs were nearly half a billion dollars in the last election cycle; they have more than doubled in 10 years. The point has been reached -- and passed -- where money distorts the political process. The average Senate seat now costs more than $3 million, the average House seat more than $300,000. Strongly contested seats can easily cost two or three times these amounts. The average senator -- not one who expects to be hard pressed -- must raise $10,000 a week every week of his term to stay abreast of these costs, which continue to rise at the rate of 20 percent per election cycle. The system is obscene.

The bill would impose spending limits. The Supreme Court has said it is a violation of the First Amendment simply to say no more than X amount can be spent. The limits must therefore be voluntary, which means that there must be inducements to abide by them. The bill makes them a condition for receipt of federal funds. Originally, the sponsoring Democrats had 80 percent of each qualifying candidate's money coming from the Treasury, with only 20 percent from traditional sources. Republicans objected, and the Democrats backed down. Now a candidate who agreed to abide by the spending limit for his state would get public money only if his opponent did not agree -- and to the extent his opponent exceeded the limit. The public funding has been backed out of the bill until it is basically an insurance policy. There would also be limits on how much any congressional candidate could take from PACs in an election cycle.

Republicans, who in recent years have been the better fund-raisers, fear the bill would strip them of their advantage. But in its present form it is a fair proposal. The spending limits would not greatly bind (the great advantage is that they would be indexed and controlled out in the future). Neither party would clearly gain; if anything, the bill would reduce the advantage of incumbents over challengers. The premium would also still be on raising private funds; that is where every candidate would start.

The Republicans can improve this bill. In some ways -- by driving the Democrats to rewrite it -- they already have. But the merits and the politics of the issue both argue against the nearsighted position they have adopted so far. On this one, they should follow the other Kansa