ONLY A FEW years ago, according to the cries of its purported victims, the National Conservative Political Action Committee (NCPAC) was about to ride roughshod across America, with roughly the same effect that the Mongol horde of Genghis Khan had over the Eurasian land mass. Now it turns out that the threat was overstated. NCPAC can point to many of its heroes in high public office, starting with the once lightly regarded Ronald Reagan. But how Mr. Reagan and his ideological comrades got there is another story.

The evidence comes from analysis by National Journal and scholar Michael Malbin of the reports that NCPAC files with the Federal Election Commission. NCPAC took in receipts of some $19.5 million for the 1984 campaign, making it the nation's largest PAC in this regard. It spent nearly $9 million on overhead. Of the $10 million it reported as independent expenditures in behalf of President Reagan's reelection, some 85 percent was spent on mailing and printing and 7 percent more on overhead and telephone. That left about $800,000, out of $19.5 million raised, to be spent on television advertising.

"A $250 contribution," NCPAC told some 15 million recipients of letters it sent out, "will pay for a minute of commercial TV time and cover the costs of sending over 120 pro-Reagan letters to voters." What NCPAC didn't tell its letter readers was that of their $250, only $10.26 would end up being spent on TV time.

What NCPAC actually was doing was using Mr. Reagan's name to build its own mailing lists. In that regard it succeeded: By sending out some 28 million letters, it increased its base of active donors from 150,000 to 250,000. In the process it lost money, raising its debt from $1 million to $4 million. That's not necessarily evidence of chicanery. Any direct mail fund-raising operation, political or otherwise, uses a lot of the money coming in to pay for more mailings; and political direct mail does badly when times are good. NCPAC's big mailings in October didn't bring in much money because too many recipients, confident that Ronald Reagan would win easily, pitched the letters in the round file.

There are two morals to this story. First, don't believe anyone who says his political enemies have an irresistible political weapon that must be blunted. NCPAC, it turns out, is not the Mongol horde. Second, look with some skepticism upon those direct-mail appeals.