TO UNDERSTAND why Paul Laxalt decided to leave the presidential race last week, consider the number 9 million. That's not the number of votes he needed; it's the number of dollars raised as of last June 30 by George Bush. Mr. Laxalt, in contrast, raised $814,000 -- a lot by the standards of not too many years ago, but not enough, in Mr. Laxalt's view, to get him to his goal of $2 million by Oct. 1. So late one afternoon he bowed out.

When the campaigning for the Republican nomination began last year, each candidate was trying to establish himself as the logical successor to Ronald Reagan. Mr. Laxalt is often called the president's best friend in politics, and has been one of his most loyal supporters since they were governors of California and Nevada 20 years ago. He won the support of some key Reagan campaign aides.

But he did not do so well among Republican contributors. Each candidate has been trying to raise money from approximately the same universe of Republican contributors, by telephone from big givers and by direct mail from the small ones. George Bush has raised $9 million -- even more, aides say, than he expected -- while Bob Dole and Jack Kemp have raised $3 million each. Against this competition Mr. Laxalt decided his $814,000 was a losing hand, and, with the suddenness and decisiveness of a Nevada poker player, folded.

The constituency of Republican money-givers, thanks to the party's efforts over the past 10 years, is a broad one -- as representative of party loyalists nationwide as the body of Iowa caucus-goers is of party activists statewide. Mr. Laxalt's supporters can argue that Republican contributors would have been wiser to have given Mr. Bush less and Mr. Laxalt more, and they can complain that Mr. Bush started earlier and with other advantages. But everyone knew the rules from the beginning.

Since the Iran-contra scandal broke, the Republican race has changed from a contest to claim the Reagan mantle to an argument over who is best qualified for the presidency in his own right. This is the right argument. It is also an argument, as Mr. Bush acknowledged when he sensibly but belatedly agreed to take part in the Oct. 28 "Firing Line" debate, that is far from ove