NEW YORK (AP) _ Domestic advertising spending is expected to fall 1.5 percent this year, the first contraction in 30 years and the biggest decline since World War II, the industry's best-known forecaster said Monday.

Robert Coen, the chief forecaster for the ad agency McCann-Erickson Worldwide, originally expected a 4.6 percent increase in domestic ad spending this year and as recently as June revised that to 3.1 percent growth.

He blamed the first decline since 1961 on the lingering economic slump and the reluctance of advertisers to revive ad spending put on hold during the Persian Gulf War in the early part of the year.

But he also conceded in an appearance before the annual PaineWebber Media conference that he was simply ''too optimistic'' about this year's spending.

Nonetheless, Coen said, he expects ad spending growth will resume in 1992, climbing 6.2 percent as the economy recovers and the elections and the Olympic games stimulate ad spending.

The increase includes an expected 3.5 percent rise in inflation, and assumes inflation-adjusted economic growth of 2.4 percent.

Other forecasters and media analysts were less optimistic about ad spending for next year, however.

Zenith Media Worldwide, a research division of the ad company Saatchi & Saatchi, told the same conference on Monday that it expects U.S. advertising spending will rise only 1 percent next year including inflation.

''We do not see 1992 as anything like a bounceback year,'' said John Perriss, chairman and chief executive of Zenith Media.

The consultant Myers Marketing & Research said its latest survey of nearly 2,000 advertisers and agency executives indicated ad spending is expected to rise 4 percent to 6 percent next year.

Barry Lucas, a media analyst at First Manhattan, said he expected growth would probably fall somewhere between the estimates from Coen and Zenith Media. Lucas said he was less optimistic than Coen that the Olympics would generate much additional ad spending but simply take spending from other areas.

Coen has been the most widely followed forecaster in the industry for years even if some of his recent estimates have proven too positive. He conceded his 1992 forecast may mean ''I'm more optimistic about the future probably than anybody in this room.''

He said domestic ad spending fell 1.5 percent to $126.7 billion this year, including a 3.4 percent decline in local ad spending and a 0.1 percent dip in national ad spending.

It was the first decline since spending dropped 0.8 percent in 1961 and the biggest drop since spending tumbled 4 percent in 1942 during World War II.

''It was a disastrous year,'' Coen said, adding that it also was atypical because of the impact of the war which started in January.

He said many advertisers reacted to the war by slashing spending plans. At the same time, it became evident that the economy was in recession and not just in a period of slow growth as many economists believed a year ago.

The spending decline was most pronounced at the local level, hitting newspapers particularly hard with declines in both classified and retail advertising that produced a 6.0 percent drop in newspaper ad revenue, he said.

Network television was also hit hard as the war and the economy derailed what appeared to be a comeback in spending by national advertisers. Coen said network TV saw ad spending fall 5 percent this year.

But revenues rose 10 percent for cable TV, 15 percent for syndicated television and 4 percent for direct mail and Yellow Pages advertising, he said.

For next year, Coen said he looks for 6.2 percent growth in spending to $134.6 billion, the biggest increases since 7.7 percent in 1988.

He said the outlook was ''even more clouded than usual'' because of the soft economy and uncertainty over whether it would contract again.

Coen expects national advertising will rise 6.6 percent next year, while local ad spending will increase a more modest 5.8 percent.

David Poltrack, head of planning and research at the CBS Broadcast Group, told the conference he expects a 5 percent increase in network revenue in 1992 in contrast to what he estimated as a decline of 3 percent this year.

But he added that his estimate anticipates an economic turnaround in the second half of the year.