President Reagan yesterday offered what he called "the closing argument" in favor of renewed military assistance to rebels fighting the Nicaraguan government by asserting that withholding aid will doom the resistance and any chance for democracy in Nicaragua.

The White House also charged that the Soviet Union appears to have renewed direct military supplies to Nicaragua's leftist Sandinista government by sending a cargo vessel to Nicaragua last month after a one-year period in which it sent supplies only through Cuba.

Democrats said they saw little new in that report and suggested the administration is trying to drum up support for its $100 million aid request for the rebels, known as contras, before a House vote scheduled the week of June 22.

Speaking to a meeting of Georgetown University's Center for Strategic and International Studies, Reagan said the aid package would give the rebels "the leverage they need to bring the communists to the table" for negotiating an end to the conflict.

If Congress refuses to supply the aid, it would "grant permission" to the Sandinista government to wipe out the contras, he said.

"Backed by a steady supply of arms from the Soviets and Cubans, the Sandinistas will be able to pin down the freedom fighters, surround them and, in time, crush them," Reagan said. "Whatever is left of free institutions in Nicaragua will be utterly destroyed."

The president again evoked the possibility of Nicaragua becoming "a second Cuba -- indeed, a second Libya" from which "tens of thousands of Nicaraguan refugees will seek to inundate our southern states."

He did not mention the reported Soviet cargo vessel, but said, "Soviet-supplied weapons in Nicaragua have mounted" so that "each day the military situation in Nicaragua twists another dangerous degree in favor of the communists."

White House officials said intelligence reports showed the Soviet freighter Sovietsky Profsoyuz left the Black Sea port of Nikolayev in April, bypassed its declared destination of Cuba and journeyed around South America before docking last month at Corinto, Nicaragua. "Our presumption," said spokesman Larry Speakes, "is that it does contain military materiel."

Officials said that was "an intelligent guess" because the ship avoided the Panama Canal, where its cargo would have to be disclosed, and because of its port of origin.

Soviet bloc nations have supplied $530 million in military aid to Nicaragua since the Sandinistas took power in July 1979, and the Soviet share of that is $240 million, according to Defense Department figures quoted by the nonprofit Center for Defense Information, a Washington-based research group.

Intelligence sources said yesterday that the quality level of Soviet aid was thought to have peaked with the introduction last year of MI24 Hind helicopters, which Reagan yesterday called "flying tanks." The quantity of arms shipments has remained steady or even declined in the past year, they said.

House Majority Leader Jim Wright (D-Tex.) said earlier yesterday that he "didn't know the Soviet Union had ever stopped providing arms to Nicaragua." He said the reported Soviet vessel might be part of the administration's "frantic determination" to find outside support for its aid package.

Government-controlled newspapers in Nicaragua did not deny the presence of a Soviet ship but said the Reagan administration was "propagandizing" and "reacting with the same old story" in bringing it up. There was no immediate official government reaction.