President Reagan is carrying two messages to the country these days, and one has the potential for getting in the way of the other.

The first, embodied in the tax-overhaul plan that Reagan considers the cornerstone of his "second American revolution," is basically leftover happy talk from his reelection campaign. It is a bipartisan message that puts dollar signs on the American dream and enables Reagan to celebrate "a golden future" and "an endless horizon lit by the star of freedom guiding America to supremacy in jobs, productivity, growth and human progress."

The rhetoric may be overblown, but the tax plan is real, bipartisan and popular. White House spokesmen gush with reports from pollster Richard B. Wirthlin showing favorable sentiment for the proposal.

But the White House is silent about its poll results on Reagan's second message, which the president delivers with a grim visage that contrasts with his cheery smile for tax reform.

The negative message is Reagan's view that Nicaragua's Sandinista government is an intolerable threat to the security of Central America and U.S. interests. It is a message he summed up two years ago by saying: "The Soviets and the Cubans are operating from a base called Nicaragua. This is the first real communist aggression on the American mainland."

Despite warnings since that 1983 declaration, Reagan has failed to persuade the House and his countrymen of the merits of military aid for the Nicaraguan rebels he calls "the moral equivalent of the Founding Fathers."

But the president does not abandon his objectives lightly. Aware that the recent visit of Nicaraguan President Daniel Ortega to the Soviet Union angered some House Democrats who had voted against aid to the rebels, Reagan delivered a attack last week on the Nicaraguan leader, describing him as "the little dictator who went to Moscow in his green fatigues."

The phrase was reminiscent of Reagan's 1976 description of Panamanian leader Omar Torrijos as a "tinhorn dictator," a remark with similar political and legislative purposes. Reagan was trying to derail President Gerald R. Ford and block the Panama Canal treaties, which were approved and virtually forgotten.

Reagan's current aim is a symbolic stamp of U.S. approval for the rebels, blessing them with U.S. foodstuffs and medicines while they buy arms from third parties. In the words of his communications director, Patrick J. Buchanan, Reagan is seeking "to persuade the country and the Congress that you have a very serious growing metastasizing cancer down there and that we ought to focus on it."

But the White House does not rush forward with public opinion polls on this issue, even though Reagan has made slight progress in persuading Americans that Nicaragua is a threat. The White House withholds these surveys because it does not want to advertise the widespread opposition to the Reagan policy in Nicaragua.

Such administration conservatives as Buchanan and CIA Director William J. Casey contend that Americans will eventually share Reagan's perception of the Nicaraguan menace. But even private CIA reports hold that the subversion of the region, regularly predicted by Reagan, is a distant rather than imminent threat. The success of his policies in El Salvador has extended the timetable.

In this context, maintenance of the rebels he calls "freedom fighters" has become a political necessity for Reagan. He boasted during the 1984 campaign that "not an inch of soil has been lost to communism" during his presidency. He has pledged, publicly and privately, not to invade Nicaragua and recognizes that he lacks support and authority for such an undertaking. His hope for sustaining pressure against Nicaragua is wrapped up in this week's expected House vote to aid the contras.

Reagan, whose persistence is a feature of his leadership, could well win his latest aid request. But victory could leave him mired in a dirty little war supporting scattered troops on which his policy has become dependent. Over the next three years, that dependence could dispel some of the bipartisan happy talk that is the long suit of the Reagan presidency.

Exchange of the Week: When White House spokesman Larry Speakes said Reagan's foreign policy rating rose after he visited a German military cemetery, Bruce Drake of the New York Daily News quipped, "The stock market went up after the crash."