A senior White House official, reflecting last week on the difficulties President Reagan faces in trying to sell his defense budget and his Nicaraguan policy to the country and Congress, recalled the old story of a dog food sales convention. A salesman tells the company president that their brand of dog food is the best-looking, best-tasting, most nutritious stuff on the market.

"So why isn't it selling?" the company president asks.

"Because the damn dogs don't like it," says the salesman.

After five years of dominating the national agenda as a premier political salesman, Reagan is running into a reluctant marketplace as he tries to sell Americans on the desirability of giving military aid to the antigovernment rebels in Nicaragua and more money to the Pentagon.

Reagan's popularity remains high, and his intensive lobbying last week appeared to move him closer to success on aid to the rebels. But the parallel debates over Nicaragua and defense spending in recent weeks have also demonstrated that his reputation as the "Great Communicator" is being severely tested over policies about which Americans are deeply skeptical.

According to many polls, they are worried about U.S. involvement in a remote jungle war in Central America, distrustful of the Pentagon's spending habits and reluctant to spend more money for either purpose while being asked to tighten their belts at home.

"The 'Great Communicator' works when he's got people emotionally involved with an issue beforehand, when he can mobilize a group that's ready to be mobilized," said political scientist Norman Ornstein of the American Enterprise Institute. "But when you have an issue the public doesn't care about -- or when the public is going in the opposite direction -- he's not going to have a dramatic impact."

Reagan pollster Richard Wirthlin said the difficult issues of defense and Nicaragua have put Reagan in a new role. "More than a salesman," Reagan has become sort of a national "teacher or mentor or coach," advocating policies that are much less appealing than tax cuts, for example, Wirthlin said.

Rep. Charles W. Stenholm (D-Tex.), a conservative who was a key member of the "Boll Weevil" faction that backed Reagan in 1981, said the defense budget and aid to the counterrevolutionaries, or contras, are "tough issues." After Reagan's nationally televised plea for new increases in defense spending Feb. 26, "the number of cards and telegrams, I could count on the fingers of both hands . . . . In previous efforts to swing public opinion, it would be in the hundreds.

"It's very alarming to me; I don't like the trends myself," Stenholm added. "The president is going to have to have a more salable game plan on defense and aid to the contras if we're going to win both of them."

Reagan himself has been complaining recently that his message on Nicaragua is not getting through to the American people -- blaming a Soviet-inspired "disinformation network" -- and he returns to the limelight tonight with a nationally televised address on behalf of the contras, his third televised speech in the last six weeks.

These appearances have become the high points of his salesmanship, going back to the address for GOP presidential candidate Barry Goldwater in 1964 that launched his political career. Many times, Reagan's first response to a big problem is to deliver a speech, according to one longtime associate, and some speeches, including his 1981 budget and tax addresses, attracted huge audiences and contributed to policy triumphs.

But experience has shown that one speech alone rarely changes public attitudes. "In most cases, Reagan began a change with one speech, and sustained it, and set a tone," said a top Republican strategist who has closely charted public trends. "With one or two exceptions, no one changes American public opinion with one speech." After five years, the nationally televised speeches have become so important to Reagan's presidency that they are now regarded as the ultimate "litmus test," as one White House aide put it, of Reagan's seriousness about a policy objective.

It has long been an article of faith among advisers that Reagan's greatest resource is his talent as a communicator, an ability exhibited not only with the big policy speeches but also at times of crisis and grief, such as his graceful remarks following the space shuttle explosion.

"He's superb when the country is feeling down," said Rep. Tony Coelho (D-Calif.), chairman of Democratic Congressional Campaign Committee and a frequent critic of the president. "He's consoling; he puts his arm around you . . . . The 'Great Communicator' has become the nation's grandfather. The public is enamored of Reagan and likes him, but has made a decision -- 'We like him but we don't necessary do what he says.' "

Defense spending and Nicaragua both seem to illustrate Coelho's point. The defense speech that Stenholm said created so little public response had no apparent impact on Congress. Last week the Republican chairman of the Senate Budget Committee, Pete V. Domenici (N.M.), proposed no increase in military spending beyond inflation, compared to the 8 percent above inflation Reagan sought.

The Reagan defense speech "didn't have an effect on anybody," said a senior administration official. "It sank beneath the waves. Did you ever see a large object sink? After a couple of minutes, there's nothing there."

One explanation offered by Reagan aides is that other news events crowded out the address -- the fall of Philippine President Ferdinand Marcos and dramatic new testimony about the space shuttle accident. Another problem, officials said, was that Reagan's address was not connected to a specific vote or action in Congress. "It was like he was on another planet," said one White House official.

But more significantly, Reagan was selling a product that Americans don't appear to be buying.

"You have a public that is up in arms about $600 toilet seats . . . and much more emotionally concerned about waste, fraud and abuse than about the Soviet threat right now," said Ornstein. "In this part, you could argue that Reagan was the victim of his own success. If they see stability, people don't want to go to the barricades for defense spending."

In the latest Washington Post-ABC News Poll, after Reagan's budget was announced, the president received the highest rating ever -- 75 percent approval -- for his handling of relations with the Soviet Union. At the same time, 56 percent of those asked said they favored "substantial" cuts in military spending to trim the deficit.

Despite this, Wirthlin said, his surveys showed that the defense speech had some impact in educating people, if not producing a flood of calls to Congress. "We accomplished as much as we could, providing a new perspective to those who listened to the whole speech," he said.

Reagan's address tonight on behalf of the contras poses related problems. Polls show that aid to the rebels has long been a hard issue to sell because of indifference and because of anxiety about U.S. military involvement in Third World conflicts.

"A substantial number of people who don't have any idea what Nicaragua is, much less where, don't see the threat," said Ornstein. "They don't see a bunch of ragtag Latin Americans on their way to Harlingen Tex. , and are probably more worried about sending their boys off to fight in some jungle."

When Reagan has tried to raise public consciousness about the issue before, he has often simultaneously mobilized the opposition to his policy; but if he failed to take the lead, not even his Republican allies would support his request for military aid to the Nicaraguan rebels.

For example, Rep. Edwin V.W. Zschau (R-Calif.), who is seeking the GOP nomination for the Senate, said he supports Reagan but finds the public on the other side. "We find among California Republicans there is a majority that opposes military aid to the contras," he said. At a recent fund-raiser, Zschau said, 300 people outside protested his support for Reagan on the issue.

"If you are going to establish public support, it will take sustained effort, and some leadership," said GOP pollster Robert Teeter. Realizing this, some White House officials at first wanted to postpone the debate in hopes that Reagan could build support over several months. But according to one top official, they were overruled by the State Department and National Security Council aides who said the contras could be wiped out by later this year if the effort was postponed.

The timing was further complicated when House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) scheduled a vote sooner than White House officials expected.

In planning tonight's address, the White House wrestled with two approaches. An early draft of Reagan's speech was filled with horror stories of Sandinista torture and murder of civilians, designed to strike an "emotional" chord among viewers. But aides said yesterday that some of these examples were discarded and that Reagan would also target his appeal on Congress, stressing bipartisanship.

"What will work best, none of us is entirely certain," said another top presidential assistant. "I'm under no illusions it will reverse public opinion. But members of Congress have been begging us to give this speech. It will create accountability. If this thing was passed on and off the stage without a spotlight, it would be much easier for Democrats" to ignore it, he added.

"The success doesn't necessarily come because people melt the switchboard. This speech can succeed in Congress even if the pollsters' numbers don't look a whole lot differently."