To enter the office of Senate Minority Leader Robert C. Byrd of West Virginia is to enter the world of Democratic doubt and indecision. Byrd has not moved. By indulgence of Republican Majority Leader Howard H. Baker J., the veteran Democrat is still in the same luxurious quarters he occupied when he was majority leader. But the feeling of power and permanence that pervaded the place until last November's election has been replaced by a palpable sense of uncertainty.

Bob Byrd, now 63, came to the House in 1952, moved to the Senate in 1958 and became his party's leader in 1976. Except for the first two years in the House, he has always been part of the majority. Like a good many other Democrats, he thought that status was guaranteed, and he cannot believe the change can be more than temporary.

"The last election," he said the other day, "was a referendum on frustration. . . . People voted for a change, and it's important we Democrats give the new president a fair trial of his policies. But that pendulum will swing back. People will tire of the far-right, ultra-conservative, single-issue politics. They will see through it soon."

Maybe. But in the meantime, Byrd and his band of outnumbered Democrats must cope with Ronald Reagan and the resurgent Republican majority -- a task for which the minority leader's previous experience offers little preparation.

Reagan, he confesses, is a puzzle. Byrd found the president's first speech on the economy "a masterpiece" of political communication and expects this week's State of the Union address to match it. But, like other Democratic congressional leaders who met with Reagan on the upcoming economic package, he found the president surprisingly "shallow" on substance.

When Reagan was asked, for example, about areas where he hoped to achieve large-scale savings, he told the congressional leaders his well-worn campaign anecdotes about the "welfare queen" of Chicago who was on the rolls with 100 different names, and about the welfare worker who showed other workers how to increase their income by quitting their jobs and going on the dole. As Byrd told the story, it was plain he was less impressed by the anecdotes than Reagan's compaign audiences have been.

But still the man and his mandate are facts of life, and Byrd is supposed to be a realist. This week, Reagan will propose his tax-and-spending cuts to Congress and the Democrats will have to respond.Byrd's rehearsal efforts at a response were more than a little bit scattershot.

"There's no question some programs ought to be cut," he said, mentioning CETA public-service jobs, legal services and food stamps as likely targets. The Senate Democrats, Byrd says, were ahead of Reagan as economizers, voting a balanced budget last spring (in an exercise that was regarded as phony by many economists and, in any event, was quickly undercut by the 1980 recession). "We have demonstrated our commitment to a balanced-budget," he declared, "and we will work with this president because when he succeeds, we succeed."

On the other hand, Byrd said, he does not believe "the cost of government can be cut 2 or 3 percent a year for the next five years, while we're improving our defense, unless Mr. Reagan goes back on his pledge not to reduce or change some of the entitlement programs."

When asked is the Senate Democrats wish to be categorized -- in the oversimplified language of us journalists -- as a loyal opposition setting aside partisanship to support the president in needed economizing, or as a dogged band, striving mightily if not always successfully to protect programs needed by farmers and city-dwellers, the aged and the poor, Byrd gave a perfect political response.

"We'll do both," he said, adding, "It's not really an inconsistency. We realize that people voted for a change. We Democrats have to readjust and reevaluate ourselves. We are not going to protect a program just because it was started by JFK or LBJ."

But, he added, in yet another of the sharp swerves in the conversation, that open-mindedness does not apply, at least in his case, to the New Frontier-Great Society programs of the Economic Development Administration and the Appalachian Regional Commission, which have poured millions of dollars into West Virginia and which Reagan has reportedly ticketed for extinction.

"Those are not make-work programs," Byrd said, in explaining why he would "resist" their destruction. "They are vital for our state to improve and progress."

That kind of politics may not be long on logical consistency, but it is typical of the improvisation needed by an opposition confronting a president whose vulnerabilities are not yet evident to the public. Byrd is learning the improvisational technique. He opposed two of Reagan's Cabinet appointees -- Alexander Haig and Raymond Donovan -- but praised many of the others. He has pressured Reagan publicly to abandon his campaign promises to lift the grain embargo on Russia and to rescind draft-registration. He supported Reagan's request to increase the debt ceiling, but he has faulted the Reagan decision to speed up oil decontrol as contributing to inflation.

It's all part of the game of touching up -- but not really tackling -- a popular new president. But it's a lot less fun than being majority leader of the United States Senate.