Rhetoric about Nicaragua intensified yesterday as President Reagan called Nicaraguan President Daniel Ortega "the little dictator who went to Moscow in his green fatigues" and as House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) warned that Reagan intends to send U.S. troops to Nicaragua.

Reagan, trying to build support for renewed aid to Nicaraguan counterrevolutionaries as Congress prepares to vote on the issue, said Nicaragua's leftist government is "a major challenge to democracy and our security."

"The little dictator who went to Moscow in his green fatigues to receive a bear hug did not forsake the doctrine of Lenin when he returned to the West in a two-piece suit," Reagan said, referring to Ortega's recent trip to the Soviet Union soon after the House rejected Reagan's request for $14 million in further aid to the contras.

"He made his choice long ago . . . . Soon, the United States Congress must make a crucial choice for our future. We had better hope that this time they choose wisely," Reagan said at a fund-raiser in Oklahoma City for Sen. Don Nickles (R-Okla.).

"It was a dark day for freedom when, after the Soviet Union spent $500 million to impose communism in Nicaragua, the United States could not support a meager $14 million for freedom fighters in Nicaragua," Reagan said. "We failed them once. We dare not fail them again."

The Republican-controlled Senate is expected to vote this week on a proposal to provide $3 million to the contras. The Democratic-led House is to debate the issue Tuesday.

O'Neill, preparing for the House battle with the administration on contra aid, said yesterday that he is "fighting to keep the American troops out of Nicaragua. It's a terrible, terrible thing for us to send our boys in because it's absolutely not needed, and I see it coming down the road.

"The administration wants to escalate the war against Nicaragua," O'Neill added. "It wants more U.S. support for the war. It wants 35,000 men in arms. It wants to raise the level of combat and the level of violence. It has mapped plans that make a U.S. invasion as easy as 'falling off a log.' "

He added, "If there were guided missiles in there, if there were MiGs in there, I would be supporting the administration."

The House is to consider a GOP proposal to provide $27 million in non-military aid to the contras. The Democratic leadership has proposed an alternative providing aid to Nicaraguan refugees outside Nicaragua and emphasizing regional peace negotiations.

After a closed Democratic caucus yesterday at which the leadership distributed a poll showing Reagan's Nicaragua policy is unpopular in the South, lawmakers said the GOP proposal has a good chance of passing in some form, in part because of anger about Ortega's visit to Moscow.