By insisting on more than their share of seats on the Ways and Means Committee, the Democrats in the House have set a notably dubious precedent. Of the 35 seats on this crucial committee, they have refused to give the Republicans more than 12 -- although the balance in the House of Representatives itself is now on ly 5 to 4 in the Democrats' favor. It's not really clear why the majority has chosen to start a difficult session with such an ungenerous, not to say churlish, gesture.

The Republicans are also irritated by the 11-to-5 division of seats in teh Rules Committee, but that's a slightly different issue. The Rules Committee is explicitly the instrument of the majority leadership, and the precise representation of the minority counts less. But in Ways and Means, where a straight partyline vote is a rarity, it matters a great deal.

Rep. Barber Conable of New York, who led the Republicans' protest against these allocations, argued that tradition would have required a ratio no greater than three seats to two. That, incidentally, is the balance in the new Appropriations and Budget committees. It is hard to see why the Democrats wanted to defend Ways and Means more strongly than Appropriations or Budget. But that was what their party-caucus decided last month, and that is what they voted to uphold when the House organized itself on Monday.

Since Mr. Reagan's first major legislation is very likely to be his tax bill, the Republicans darkly suspect an attempt to derail the new administration's economic policy from the beginning. That will probably turn out to be an exaggeration. It's beginning to look as though there will be a rapid bipartisan compromise on a tax bill closely resembling the one that the Senate Finance Committee drafted last summer. Perhaps there won't be much combat on tax policy after all. Perhaps the Democratic lack of charity in the matter of the Ways and Means seats arises from nothing more substantial than the leadership's fear that a conservative coalition might seize control of the committee's wide powers later in the session. But this memorably greedy division of seats will certainly be imposed on the Democrats themselves if they should lose control, however narrowly, in the next Congress.