Fundamental differences in the attitudes of Republicans and Democrats toward the Soviet Union, views that helped to define the parties in recent decades, have largely vanished, according to a major poll released Thursday.

The national survey of 1,000 registered voters also showed that public support for the proposed Intermediate-Range Nuclear Forces (INF) Treaty has increased since October, and that Soviet leader Mikhail Gorbachev is more popular than ever with Americans.

But the survey disclosed that the public remains suspicious of moving too quickly on further nuclear arms reductions and found that large numbers of Americans remain distrustful of the Soviet Union.

Researchers said they were surprised by the generally consistent agreement among Republicans and Democrats on the Soviet Union.

"I was struck by the absence of a difference between the parties," said Fred Steeper of Market Opinion Research in Detroit, which conducted the poll. "As late as '83 and '84 there were huge differences in attitudes. It's due to {President} Reagan's change of policy, and the Republicans are following him."

Among the areas of agreement:

62 percent of Republicans and 66 percent of Democrats said the United States can trust the Soviet Union at least somewhat.

71 percent of Republicans and 74 percent of Democrats said they think that the United States can trust Gorbachev.43 percent of Republicans and 44 percent of Democrats said the Soviet Union "can be trusted" to keep its part of the INF Treaty.

The survey did show that Republicans are somewhat more suspcious of Soviet intentions. Just over a third of all Democrats (34 percent) and 45 percent of all Republicans said the Soviet Union is "mainly interested in world domination."

Voters remained cautious but generally positive when asked whether they favor treaties that would make deeper cuts in the Soviet and U.S. nuclear arsenals.

The survey showed that voter support for the INF Treaty has increased from 72 percent in October to 79 percent in the January survey. But only about one of four -- 27 percent -- said the treaty is "very significant" while 55 percent said it is moderately significant.

Gorbachev remains popular with Americans, according to the poll. More than seven of 10 (72 percent) said they have a favorable impression of the Soviet leader, a 6 percentage point increase from the October survey. But Gorbachev's support remains soft, the researchers said: Only 21 percent of those surveyed said they had a "very favorable" view of him.

The poll also showed that Americans have little knowledge of basic elements of the INF Treaty. For example, fewer than 10 percent of the public knew that the treaty would eliminate all the U.S. missiles in Europe that can reach the Soviet Union.

The findings came from the second in a series of surveys in the "Americans Talk Security" series sponsored by Massachusetts businessman Alan Kay.

The survey was conducted by telephone Jan. 7-14. Margin of sampling error for the overall results is plus or minus 3 percentage points.