VENICE -- Ronald Reagan may have 18 months left in his presidency, but his dominant international role appears to have ended abruptly at the summit in this beautiful city, the seventh he has attended.

What Reagan discovered was that the same people who snapped to attention when he was at his peak are now merely snapping at him.

"He's showing his age," said one who had a chance to observe him closely.

Wounded by the Iran-contra scandals, and defensive about America's weak economic performance, Reagan was subdued and unimpressive. It was a sharp contrast with his near-total control of the Tokyo summit and its agenda last year.

Normally eager to brief reporters in an effort to drown out the other nations, the American side in Venice sought seclusion, while attempting to convince the world of Reagan's continuing strong leadership by saturating the TV talk and interview shows.

The blitz didn't work. By this reporter's count, four key officials, in a White House-directed barrage, did 27 TV interviews -- 12 of them by Treasury Secretary James A. Baker III alone -- in three days, while they ducked the regular press.

Meanwhile, other delegations, especially the Canadians, Japanese and French, were making news with prompt briefings and advisories.

To some extent, the decline of the Reagan presidency may be rubbing off on Baker, who made waves in Tokyo last year with a new initiative to coordinate international economic policy.

But this year, coordination seems to be mostly empty rhetoric as the Americans struggle for more stimulus to economic growth from Europe, while Europe says "no" and demands instead real American action to cut its budget deficit.

German Chancellor Helmut Kohl, after a meeting with French President Francois Mitterrand, said bluntly that the U.S. budget deficit "is the central problem" for the global economy, and that the two European leaders weren't satisfied with the boring American assurances that the deficit is being curbed. Special friends Yasuhiro (Yasu) Nakasone and Margaret Thatcher also joined in criticizing American policy.

The notion of West Germany as a "locomotive" for global growth -- first suggested by former president Jimmy Carter at the 1978 Bonn summit -- is finished, so long as he is chancellor, Kohl said.

At one of the plenary sessions, German Foreign Minister Hans Dietrick Genscher put it this way: "Germany will be a locomotive for stability, not a locomotive for inflation." Germany thus fended off pressures from the United States and others to reiterate its pledge at the Paris meeting last month to expand its economy if growth slips.

So what we have is a dialogue of the deaf, in the face of high unemployment, huge trade and current account imbalances, Third World debt and the prospect of the dollar under continuous siege. The world economy needs some help, and its leaders can't agree on how to supply it.

It was this perception of a near dead-end on multilateral economic innovation that persuaded Deputy Secretary Richard Darman to leave his post several weeks ago, and a question frequently asked at Venice is whether Baker can be far behind.

This is not to say that America's six major partners do not have their own economic and political problems; their heads of government have not yet shown they are ready to seize the reins of global leadership. Europe is still far behind America and Japan in getting commercial market benefits from the technological age.

Japan is in the midst of economic adjustment, and Nakasone could be ending his term in October. But his relative success in Venice, where he detailed a fiscal expansion package and an offer to recycle $20 billion in new money to the Third World, may buy him another year. At a minimum, the Venice summit strengthens Nakasone as a power behind the scenes in Japan. He could be the first Japanese premier to return to office at another time.

"Believe me, he's thinking of it," a top Japanese bureaucrat said.

Although neither Japan nor West Germany is a political superpower, ready to grapple with the Soviet Union, their over-all economic strength puts them in a better position to press Reagan in many ways.

As my colleague Jim Hoagland reported earlier, the Europeans, led by British Prime Minister Thatcher, felt at ease pushing Reagan at the opening dinner to explain how his negotiations with Soviet Premier Mikhail Gorbachev will affect the Atlantic Alliance's military strategy. They weren't impressed by the depth of his responses.

The defensive White House response in Venice was to keep Reagan under cover -- almost literally -- until his post-summit news conference. They even insisted on full canopies from villa or hotel exits used by Reagan, extending to the canal docks, so that no one could get a view of the president.

By contrast, Kohl played the role of relaxed extrovert, appearing more than once in St. Marks square to enjoy a drink and the local scene.

Despite the Reagan team's full court press on ABC, CBS, NBC and CNN to convey a different picture -- one of a Reagan fully involved -- the television show that drew most attention among the press corps and officals was Fawn Hall testifying at the Iran-contra hearings on how she shredded papers for Col. Oliver North.

When Secretary of State George P. Shultz made his one appearance in the White House press room, Fawn Hall was still holding forth on the tube, thanks to CNN. A White House aide hastened to throw the switch. He said he had his orders.