The U.S. merchandise trade deficit jumped to $16.5 billion in January, as American manufacturers continued to lose ground to imports, the Commerce Department reported yesterday.

The January trade deficit set a monthly record and was $1.4 billion higher than the $15.1 billion December level and $4.1 billion greater than the monthly average for last year.

The continuation of record trade deficits showed that the declining dollar has failed to arrest the erosion in America's international competitiveness that has allowed imports to overwhelm exports for the past three years. Even the traditional U.S. trade surplus in agriculture of $311.7 billion in January was only slightly more than half of December's surplus.

Nonetheless, administration officials said better days are ahead on the trade front. White House spokesman Larry Speakes cited the drop in world oil prices, economic growth and the lower value of the dollar as factors that "will begin to have an impact in the second half of 1986."

"We should not expect a real improvement in the trade balance until the last half of this year," Commerce Secretary Malcolm Baldrige cautioned. "It will take that long for the impact of the lower dollar to shift demand toward American goods."

In congressional testimony this week, Baldrige predicted the 1986 trade deficit would be the same, or slightly less, than last year's record $148.5 billion.

C. Fred Bergsten, director of the Institute for International Economics, agreed that the trade deficit would stop growing, which he said will be "quite a step, because the deterioration has been so great." But he added that the dollar will have to drop another 15 to 20 percent this year to bring America's trade picture near a balance by 1988.

In his testimony this week, Baldrige said the trade deficit with Japan -- the largest the United States has with any country -- is likely to "hover" around $50 billion. Japan ran a $5.5 billion surplus with the United States in January, $700 million greater than in December.

While congressional pressure for protectionist legislation appears to have waned slightly with the sharp fall in the dollar over the past year and the promise of lower trade deficits in the future, Japan's continued large surpluses, along with reports that it is offering subsidized low-interest loans to import-oriented businesses hurt by the falling dollar, may spark passage of legislation aimed specifically at its trade practices.

"If that deficit with Japan does not come down below $50 billion, you are going to see a Congress in a protectionist mood take action," said Rep. Don Bonker (D-Wash.).

U.S. Trade Representative Clayton Yeutter and an influential lawmaker on trade issues, Rep. Sam Gibbons (D-Fla.), moreover, warned Japan on a Chamber of Commerce telecast this week that the subsidy program will bring retaliation from the United States if it acts against the falling dollar.

The high dollar has been blamed for a major share of America's rising trade deficit by making foreign goods less expensive in this country while increasing the cost of U.S. products in overseas markets. The dollar has dropped more than 20 percent overall and about 30 percent in comparison with the West German mark and the Japanese yen.

Imports in January reached a record $33.47 billion, an 11.7 percent increase over the 1985 monthly average, paced by manufactured goods worth $23.5 billion. These included $3.7 billion in new cars, half of them from Japan; $1.6 billion in telecommunications equipment; $1.5 billion in electrical machinery, and $1.5 billion in clothing.

Exports in January totaled $17 billion, about the same as the month before, but 4.5 percent less than the 1985 monthly average.

American companies sold $11.4 billion in manufactured goods overseas, half the value of foreign manufactured products brought into the United States, a reversal of this country's traditional advantage.

In addition to its deficit with Japan, the United States ran deficits in January of $3 billion with Western Europe, $1.8 billion with members of the Organization of Petroleum Exporting Countries, $1.7 billion with Canada, $1.2 billion with Taiwan, $545.2 million with Hong Kong, $529.9 million with South Korea and $498.8 million with Mexico.

The Commerce Department revised downward the December deficit, which was reported as $17.4 billion on a seasonally adjusted basis.