President Reagan pressed yesterday for military aid to Nicaraguan rebels with an emotional claim that he is "a contra, too," and White House officials agreed with House Speaker Thomas P. (Tip) O'Neill's estimate that the administration is 12 to 14 votes short of victory in the House.

White House chief of staff Donald T. Regan said in an interview that he did not dispute the Massachusetts Democrat's tally. Another White House official said the administration has "steadily chipped away" in the House, where a vote on the package of $70 million in military aid and $30 million in nonlethal aid is scheduled Thursday. But the official added, "The last 15 votes are going to be the hardest to get."

The administration sought yesterday to sway the votes of undecided lawmakers by emphasizing its commitment to a peaceful resolution of the Nicaraguan conflict and to regional stability. An eight-page report to Congress on "Freedom, Regional Security and Global Peace" stressed U.S. opposition to dictatorships of any kind.

Calling for democratic solutions in the developing world, the report said, "In this global revolution, there can be no doubt where America stands. The American people believe in human rights and oppose tyranny in whatever form, whether of the left or right.

"We use our influence to encourage democratic change, in careful ways that respect other countries' traditions and political realities as well as the security threats that many of them face from external or internal forces of totalitarian- ism."

A senior official who briefed reporters called the report "half analysis and half prescription" and said it represented "nothing new" in the way of policy. He said that it had been in preparation for several weeks but that the timing of its submission to Congress was "clearly related" to the administration's efforts to obtain the aid package for the Nicaraguan rebels.

In contrast to the bland and diplomatic language of the report, the president expressed passionate support for the cause of "the freedom fighters, the so-called contras" in a speech in the Old Executive Office Building to backers of the aid package.

Explaining that "contra" was short for the Spanish word for "counterrevolutionary," the president said it was an accurate definition because the Sandinistas who toppled Nicaraguan dictator Anastasio Somoza in 1979 had turned their revolution into "a communist coup."

Reagan said, "So I guess in a way they are counterrevolutionary and God bless them for being that. And I guess that makes them contras and so it makes me a contra, too." The crowd burst into applause, mixed with some laughter.

In the past Reagan has compared the contras to America's founding fathers and to the Hungarian freedom fighters who were crushed by the Soviet Union in 1956. Yesterday he compared his proposal for aiding the rebels to the Lend Lease program through which the United States provided armaments to Great Britain to fight Nazi Germany during World War II.

The president likened the Sandinistas to "Murder Inc.," a gangland mob of the 1930s, and called them "players in a drama whose aim is to spread communism throughout the hemisphere."

Reagan denounced what he called "slanders" against the contras, among them the claim that contras were incapable of stopping the Sandinistas militarily. The president called this a "self-fulfilling prophecy" and said if the contras are given "nothing but a pat on the back and a roll of bandages" they would probably lose.

"Well, the truth is there are over 20,000 freedom fighters who are desperately waiting for everything from shoes to ammunition," Reagan said. "And when they get them, they'll move. And when they move, they'll win."

Expounding a theme he is expected to return to Sunday night in a nationally televised speech, Reagan said, "The Sandinistas will come to the negotiating table only when they see the carrot of peaceful settlement backed up by the stick of a well-equipped armed opposition, and nothing is as urgent as the question of Nicaragua. There is no question . . . more crucial to our future than what happens in Central America."