political party cannot last long if it insists on rigid positions that are shunned by most voters.

So from an entirely practical, self-preservationist viewpoint, the Republican national chairman, Jim Nicholson, is right to chide Senator Bob Smith of New Hampshire, who is expected to quit the party today because some Republicans are less than absolute in their opposition to abortion and gun control.

At the same time, political parties will continue to lose relevance if they cannot at least summon enough strength to protect their own organizational integrity.

Voters can be excused if they look skeptically at Smith's move. In ignoring the advice of many friends and deciding to run for president, Smith willingly took on a long-shot crusade partly to advance his conservative positions.

Increasingly, however, it has become clear that his candidacy was not catching on, even in his own state, home of the nation's first primary. The prospect was that as a Republican, Smith might get very few votes, possible even fewer than some of the fringe candidates. This would be an embarrassment to him and would undercut his message.

Nicholson was correct on writing Smith to say that his defection would be ''a serious mistake'' and potentially counterproductive.

For one thing, if Smith continues to seek the presidency as an independent candidate, he might still receive an embarrassingly small number of votes, and most of those would probably hurt the Republican nominee more than the Democrat.

The fact that several other Republicans, including Steve Forbes, Pat Buchanan, and Gary Bauer, have criticized Nicholson - Forbes accused him of ''running people out of the party'' - only shows that the ideologues would rather fight each other than win votes.