President Reagan last night accused the Democrats of seeking to "dismantle" his administration's defense buildup and called upon the Republican Party to "stand a lonely vigil to guard the gates of freedom."

In remarks at a GOP congressional dinner, the president used blunt and partisan words to defend his military budget and the tax overhaul bill approved by the Senate Finance Committee.

"The other side still seems to have only two ideas: raise taxes and slash defense," Reagan said. "And they still talk about America's defense as if it were somebody else's special interest. Maybe there's something revealing in that."

Reagan said the Democrats had refused to address the "one overriding question" of what he called the "enormous and dangerous military buildup" of the Soviet Union.

In a reference to the House-passed version of the budget, Reagan said he would not "permit some reckless antidefense budget to tear down all that we have done."

The president's speech defended the record of his administration and raised issues that were the cornerstone of his campaign against President Jimmy Carter six years ago.

"In 1980 and 1984, the American people have repudiated the Democratic Party's politics of envy at home and weakness abroad," Reagan said. "But don't think for a moment that the other side has been chastened . . . . Given half a chance, they would quickly begin to dismantle the strong and proud America that we've spent these 5 1/2 years building. They'd gleefully take it down, piece by piece, all the while talking on and on about their so-called 'fairness.' "

Reagan also called anew for military aid to the anti-Sandinista rebels in Nicaragua and said that "the Democratic House leadership has twisted the rules so that a fair up-or-down vote on aid to the Nicaraguan freedom fighters was not possible."