A few hours after George Bush scored his biggest win of the year over Ronald Reagan, ABC News opened its special election-night "Nightline" report with these words: "ABC News projects that Ronald Reagan has now gained enough delegates to clinch the Republican nomination for president."

At the same moment, CBS News was reporting that its delegate count showed the same thing. NBC, which uses a different system of counting delegates, did not report that Reagan had clinched the nomination that night, but it did predict -- before any of the results of Tuesday's primaries were known -- that on the basis of a nationwide survey of the Republican race, Reagan would end up with an enormous majority over Bush when the delegate-selection process ends in July.

Among the millions of people watching those reports was George Bush, who was twirling the dials at his motel in Canton, Ohio, with a sinking heart. "I just went from a feeling of exhiliration to a feeling I hadn't done anything," he said afterward, and nine hours later, at breakfast yesterday, he announced that he would reassess his determination to say in the race.

Bush might have drawn some solace had he known that the delegate counters at some other major news organizations, including The New York Times, The Washington Star, and the two major wire services, were reporting that Reagan has not yet won enough delegates to take the nomination. But none of those outlets could compare with the clout and reach of the television networks.

In many ways, the question of how many delegates Reagan controls right now is academic, because just about everybody -- including George Bush -- agrees that Reagan will have enough delegates in the remaining 12 primaries to put him well over the required number.

But to Bush and his partisans, the question cuts to the quick. His strategy for coming from behind to capture the nomination requires a strong showing in the major upcoming primaries -- and then peeling away delegates supporting Reagan but not legally bound to him.

Bush did not directly challenge the accuracy of the ABC and CBS delegate projections. And among the news organizations engaged in delegate-counting, none criticizes the accuracy of competitors' conflicting counts.

The major reason for discrepancies among the various counts is a difference of the opinion about what constitutes a "firm" delegate.

NBC News, which has had the most conservative counts of all major news organizations this year, considers a state's delegates "firmly" allocated only after the selection process is complete.

CBS News, in contrast, counts a delegate as "firm" as soon as it seems clear to the network's reporters and researchers that the allocation of the delegates is certain. This may occur before the delegates are finally chosen. d

Iowa is an example -- there are more than a dozen states like it -- where these two systems produce conflicting results. Iowa's delegates to the GOP national convention will not be finally chosen until the state convention on June 6. Conservative counters, therefore, do not include any Iowa delegates in their totals.

But the choices the state convention will make have already been determined by precinct and county caucuses where Bush and Reagan supporters battled to win delegates to the state convention. The results of those caucuses are public, and from them one can tell with some certainty how the state convention is going to turn out. So some delegate-counters include the Iowa delegates.

Another source of variation in the counts is the treatment of "uncommitted" or "undeclared" delegates. Some are "uncommitted" only on paper; a counter who bothers to call may find that the delegate has in fact decided which candidate to support at the national convention.

Some of these delegates, though, apparently give different answers to different news organizations. Thus among the Republicans in Pennsylvania, where a big bloc of delegates is officially "uncommitted," no two delegate-counts agree on the exact breakdown among backers of Reagan, Bush, John B. Anderson and "undecided."