When Sen. William L. Armstrong (R-Colo.) came to Washington, he set out to "drain the swamp, not make friends with the alligators."

In the four years since, while his zeal has sometimes enraged fellow swamp-drainers as well as the alligators, Armstrong has emerged as a formidable if somewhat quixotic force among conservatives in Congress, and is poised for a leading role in the imminent congressional debate on Social Security.

Still in his first term in the Senate, Armstrong has positioned himself for power as a member of three of the four major fiscal committees (Finance, Budget and Banking), has earned a reputation as a serious-minded gadfly and is already being touted for president by some leaders of the New Right.

The brainy, articulate and media-conscious lawmaker has challenged not only Democrats but leaders of his own party, including President Reagan, often with a sharp-tongued but amiable wit that has drawn admiration along with irritation.

And now, as chairman of the Senate Finance Committee's subcommittee on Social Security, he is planning to take them all on at once in an effort to revamp the rescue plan for the huge retirement system, which tops a heavy legislative agenda in the 98th Congress.

"The odds are less than 50-50, but we have a real fighting chance," said Armstrong, clearly relishing not only the scent of another uphill battle but also the attention that his latest crusade is getting.

In both tactical acumen and intellectual grasp, the boyish-appearing Armstrong, 45, is widely viewed as one of the brightest of the new breed of Republican conservatives elected to the Senate over the last four years.

But some say he may be too bright, brash and even bull-headed for his own good--a "pain in the neck," one colleague has called him. And, as a relentless conservative from a politically schizophrenic state, he faces a potentially difficult reelection campaign in 1984.

"He always wants to be a pusher, a gadfly, a breaker of new ground. He could charm the tongue out of a snake . . . . But he's never gotten out of his minority mold, and I'm not sure he knows how to get reelected in Colorado," said a congressional staffer who is both an admirer and a critic.

Said another: "Senators run hot and cold toward him. They respect his insight, his principles. But a lot of the time it seems that principle gives way to zealousness and a desire to get the limelight . . . . People hold him at arm's length because they never know when he'll pop off."

But Armstrong is not just a Jesse Helms in a Boy Scout suit.

Unlike the North Carolina senator who pushed the Senate beyond its limits of endurance by filibustering against the administration-backed increase in the gasoline tax last year, Armstrong--perhaps profiting from Helms' excesses--has made it clear in advance that he won't stand in the way of final action on the Social Security plan.

He has reassured colleagues and the public, in private conversations, news conferences and interviews, that he is not out to kill or delay passage of the measure.

He will only seek to "improve it," he says, by shifting the emphasis from tax increases on both workers and retirees to finding ways to cut the cost of benefits, and by pushing for more long-term savings. He favors, for instance, raising the retirement age after the year 2000.

"I'm not trying to kill the package," Armstrong said in a recent interview. "I'm just trying to amend it. I'm trying to improve it. I'm not going to filibuster, and I don't have any intention to slow down passage of it. In fact, I've been arguing for a year and a half that we've waited too long already. But I do desire to make it a more balanced package."

But lest those words be taken as a sign that Armstrong has gone soft, he states his case against the Social Security plan in these more characteristic words that underscore how serious he is about the Social Security fight and how prickly the debate may be:

"You can fight city hall, and once in a while you'll win, especially on an issue like this," he said with the kind of cool passion that has become his oratorical trademark.

"The reason this is more than a run-of-the-mill issue is that it affects in a very serious personal way nearly every American--36 million recipients, all the rest of us as payroll taxpayers, every family in America in more than one way. And none of these families was in on the deal. It was an insiders' deal, with no effective consultion with the people who are affected."

In three terms in the House and two-thirds of a term in the Senate, Armstrong has strained his colleagues' patience with his assorted crusades, including a recent one, against tax breaks for members of Congress, that prompted testy and unusually personal, un-Senate-like references to the fact that Armstrong, a millionaire business entrepreneur by his mid-20s, doesn't need to worry about job-related living expenses.

At another point, not long after Reagan submitted his first budget, Armstong embarrassed the president by leading a backbenchers' rebellion on the GOP-controlled Budget Committee, contending that Reagan's plan was too vague on long-term savings aimed at reducing deficits.

At another point, he threatened to hold up a debt-extension measure in order to give Reagan greater purse-string powers, even though the White House was adamantly opposed to his timing out of fear that the debt legislation, essential to keep the government operating, might be jeopardized.

Armstrong's efforts in the Senate, coupled with the fact that he declined to back Reagan over former president Gerald R. Ford for the Republican presidential nomination in 1976, have made the White House wary of him, although it is reportedly pleased with the restraint with which he has handled his Social Security revolt.

Armstrong, in turn, insists that the president's budget problems would be less difficult now if Reagan had heeded his advice to go to the mat early both to ensure future savings and to get additional powers to cut spending.

But, despite Armstrong's maverick behavior, a Helms-style effort against the Social Security package would be dangerous for him on several counts, even if not, as Armstrong implies, out of character.

For one thing, just as filibusters were becoming almost routine in the Senate, Helms gave them a bad name. For another, Social Security is serious business, a kind of political tar baby that one approaches with extreme caution.

Besides, Armstrong contends that he's a conciliator at heart, a claim that may surprise some of his colleagues.

More likely, as a Senate aide who's watched him over the two years since Republicans took control of the chamber suggests, Armstrong is torn between being a lone ranger and a leader, perhaps, at this stage, trying to be both.

Armstrong puts it this way:

"The role I play in the Senate is one which does not come to me instinctively. I came into the Senate after six years in the House, where I was in the minority in a very undemocratic institution, in an institution where, if you don't have your elbows out and fight every step of the way, they won't let you do anything . . . .

"So I came to the Senate and found in some respects a quite similar situation because I was in the minority, but there was a much broader opportunity to register points of view.

"That isn't my natural personality. My natural personality is to get along with everybody."

As Armstrong remembers it, he was "usually the guy who who put the deals together" when, before coming to Congress, he served as majority leader of the Colorado Senate. And he points with pride to the occasions when he has done so in Washington, noting his recent shepherding of a Social Security disability bill through Congress.

Moreover, he says, he figures his position and role in the Senate has changed since it became more conservative. "When I came on the Senate Budget Committee, there was virtually no one speaking up for the point of view that I felt needed to be considered.

"I got only one or two votes at the start, but the next year I began to prevail on very similar motions. What happened was there was a change in Congress. My role around here has been a much more accommodating role than it was two years previous. The point of view that I'm interested in is much closer to the middle of the road than it was--the road has moved."

Armstrong smiled, and added, "Sure, maybe I've moved a little, too. The point is that the role a senator plays depends on the issues and circumstances and what other people are doing." As he sees it, he says, "In order to be an effective compromiser you've got to be willing to fight . . . . If you're not willing to fight, you have nothing to bring to the table."