IN A FEW days or weeks, Congress may be staring an awkward issue in the face. The issue is what to do about misconduct by President Reagan, and it seems safe to predict that Congress will blink.

The idea of impeaching Ronald Reagan seems preposterous, even to his sharpest critics. It has the flavor of regicide, or patricide, or shooting a lovable old dog. Reagan remains an immensely popular president, who is genuinely liked and admired by the American people. Whatever his mistakes or misjudgments in the Iran-contra affair, we aren't likely to impeach the man. The Gipper may not be perfect, but he isn't Richard Nixon!

That simple political fact of life -- that impeachment of Ronald Reagan is virtually unthinkable -- poses a fundamental problem for the congressional investigation of the Iran-contra affair. The problem is that as the investigation draws closer to the issue of whether the president approved lying and illegality by his aides, it is heading toward a confrontation that Congress isn't prepared for, and one that it almost surely will lose. David Ignatius, an associate editor of The Washington Post, is editor of the Outlook section.

You could see the look of defeat on the faces of the members of the Iran-contra panel last week as they listened to Lt. Col. Oliver North's forthright defense of his actions. North was boasting about lying to members of Congress and shredding documents under the noses of Justice Department attorneys, and senators were rushing to the network interview booths to say what a persuasive witness North had been.

Congress won't do nothing. The Iran-contra committees undoubtedly will issue a blistering final report criticizing the president. Congress may also pass a new statute intended to bring covert operations more firmly under congressional control. In addition, special prosecutor Lawrence Walsh will probably obtain more indictments. But none of these steps will go to the heart of the matter -- which is to deter abuse of presidential power. There's one obvious and emphatic remedy for that, and we aren't ready to contemplate it.

Because Congress is going into battle carrying the legal equivalent of a popgun, it's possible that the Iran-contra hearings may accomplish the opposite of what was intended. Rather than checking presidential ability to conduct "off-line" covert operations in defiance of Congress and the law, the investigation may end up demonstrating that a popular president can flout the will of Congress and get away with it -- so long as his aides destroy enough documents.

For Ollie North, that would be the ultimate victory: A demonstration that the executive branch does indeed have the authority to conduct "deniable" intelligence operations, even to the point of lying to Congress and obstructing justice, and that the legislative branch is powerless to do anything about it. It would amount to a congressional acceptance, by default, of North's premise that, "By their very nature, covert operations. . .are a lie."

Whether we reach these troubling constitutional questions depends on the course of the hearings over the next few weeks -- and what evidence they yield about President Reagan's role in the scandal. Issues that today seem sharp in the abstract may blur in the fog of conflicting testimony and the president's faulty memory. But consider the following scenario: For all North's personal charisma, his testimony last week took the Iran-contra scandal one step closer to the Oval Office. North testified that he had sent "up the line" toward President Reagan approximately five memos that mentioned the diversion of Iran arms-deal profits to the contras. North said he gave the memos to his immediate superior, national security adviser John Poindexter, and doesn't know if the president ever saw them. But until a few days before he left the White House last November, North said, "I assumed that the president was aware of what I was doing and had, through my superiors, approved it." Admiral Poindexter's testimony this week may carry the scandal across the threshold and into the Oval Office itself. Nobody can be sure what Poindexter will say. But one member of the Iran-contra committee who listened to Poindexter's private discussion with committee investigators several weeks ago says his public testimony will be "very, very explosive."

A hint of how explosive it could be appeared in a March 8 story by Washington Post reporters Bob Woodward and David Hoffman. They wrote that Poindexter, at least initially, "planned to construct a legal defense around his contention that on two occasions in 1986, he told Reagan that the arms sales to Iran were generating money for the contras . . . . Poindexter did not tell Reagan there was an illegal diversion of money, but rather that funds for the contras contributed by Iranians or Israelis were 'an ancillary benefit' of the arms sales."

And what then? If Poindexter does indeed testify this week that he told Ronald Reagan in general terms about the diversion of funds, what will Congress do about it? What remedies does Congress have to deal with allegations of serious misconduct by a sitting president?

Ultimately, it comes down to one awesome weapon. Rep. Lee Hamilton, the chairman of the House committee investigating the Iran-contra affair, explained last month what would happen if evidence emerged that Reagan had approved one of North's "smoking gun" memos outlining the diversion of funds. "If that occurred," Hamilton said on ABC News, "you would have a demand for impeachment proceedings."

It probably won't happen that way, of course. By North's own account, the relevant documents that might have carried the president's initials approving the diversion of funds all have been shredded, so there probably won't be hard documentary evidence. The question will be left to the hazy realm of memory: John Poindexter's memory, Don Regan's memory, Ronald Reagan's memory.

And memory, as every lawyer knows, is an imperfect instrument. Richard Nixon himself admonished his aides in one of the most famous Watergate tapes that when called on to testify, "You can say, 'I don't remember.' You can say, 'I can't recall. I can't give any answer to that, that I can recall.'" Watergate, which was deadly serious from the beginning, provides a useful benchmark for our national lack of seriousness in the Iran-contra affair. From the first days of Watergate, impeachment loomed in the background -- rarely mentioned at first, implausible until Spiro Agnew's resignation, but always an issue. Not so with the Iran-contra affair.

The Watergate investigators lawyers understood that the fundamental issue before them was whether the rule of law would prevail in our democracy. Soon after a special prosecutor was appointed to investigate the scandal, a young attorney began preparing a memo that eventually ran to 128 pages, analyzing the crimes that might have been committed by Richard Nixon.

As the evidence of presidential knowledge of the Watergate coverup mounted, the prosecutors debated whether to indict Nixon as a co-conspirator or instead provide the evidence of presidential wrongdoing to the House of Representatives, for impeachment proceedings. Special Prosecutor Leon Jaworski decided that impeachment was the only proper course.

Impeachment was excruciating for members of Congress, even in the supercharged atmosphere of Watergate. In their book "The Final Days," Bob Woodward and Carl Bernstein quote from the tape-recorded diary of one member of the House Judiciary Committee, Rep. Caldwell Butler, who said of the impeachment vote: "I was never more moved by any experience that I've had. I'm quite sure that if I hadn't been in public view I would have cried or shed a few tears."

The Iran-contra affair isn't Watergate, as we keep reminding ourselves. The issues involved are far more subtle, and the indications of presidential misconduct are far hazier. But the bizarre nature of the Iran-contra story may be blinding us to the seriousness of the issues it raises about the abuse of presidential power.

What Lt. Col. North was asserting last week was that in conducting covert operations, the president and his men are above the law. They have a right to lie, when they deem it necesssary, to protect the secrecy of their covert operations. When weighing lives against lies, North implied, lives should be paramount. Legal details -- like obeying the laws passed by Congress -- are secondary.

North disclosed Friday that the president's men hoped to make the the Iran-contra affair a model for future operations. He testified that the president's friend and adviser, CIA Director William Casey, had proposed creating a permanent structure, "self-financing, independent of appropriated monies and capable of conducting activities similar to the ones that we had conducted here."

That assertion of executive power goes well beyond the claims made on behalf of Richard Nixon. In arguing the Watergate tapes case before the Supreme Court in July 1974, Nixon's own lawyer, James D. St. Clair, conceded: "The president is not above the law. Nor does he contend that he is."

North seemed to be challenging that premise last week. His testimony amounted to a plea for a special category of presidential authority that would apply to covert operations and supercede the normal rules of law. He was blunt in his testimony on Tuesday: "Part of a covert operation is to offer plausible deniability of the association of the government of the United States with the activity. Part of it is to deceive our adversaries. Part of it is to ensure that those people who are at great peril carrying out those activities are not further endangered. All of those are good and sufficient reasons to destroy documents."

Many Americans appeared last week to support Ollie North's pragmatic view of the law. Indeed, if the polls are right, the public believes that Ronald Reagan personally approved the diversion of funds to the contras, despite his repeated denials. Yet there's no thought of impeaching a president the public evidently suspects of lying. Congress shows no sign, either, that it's prepared for a bloody constitutional confrontation.

So the surprising bottom line of the Iran-contra affair may ultimately be the acquiescence of a bootless Congress to the proposition that intelligence operations -- which usually involve systematic violation of the laws of other countries -- are sometimes outside the legal framework of our own nation, too. That sort of double standard applies in most countries, where legislators routinely wink at covert operations, or protest them publicly while quietly accepting them as a fact of life. But this cynical approach to government would respresent a stunning change for the United States.