Four years after being crowned king of Capitol Hill, President Reagan has become more of a reigning monarch than a ruling prince.

He still sets the agenda for Congress, keeping it within generally conservative parameters. He fends off direct challenge with a commanding personal popularity that makes lawmakers of both parties wary of hand-to-hand combat. And, as House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) has noted, he has a powerful resiliency that enabled him to rebound from earlier low points.

But despite his landslide reelection to a second term only six months ago, as Reagan prepares to make tax simplification his new top priority, he finds himself a considerably less imposing force in the Capitol than he was four years ago.

"He's still popular, but the issues he's taken on are not as popular," said Rep. Thomas J. Tauke (R-Iowa). "Also, he's taken on a lot of these tough positions at the same time . . . . And he's been to the well a lot of times. In the beginning of his presidency , when he asked, you did it to the extent you could. When he comes back for the 22nd time and you backed him 19 or 20 times, you're less concerned about breaking with the president. You're also less concerned you'll harm his presidency."

Reagan "changed the rules of the ball game; we're now playing by his rules," said Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee. "But he's no longer the star player. He can't change the rules any more. He can't push the revolution."

The president has been impeded by his lame-duck status, by signs of economic slowdown and by missteps in the once sure-footed White House lobbying operation. Replacing the single-minded drive for tax and spending cuts four years ago is a clutter of priorities ranging from deficit reductions to Nicaragua, some of which do not have many ardent friends in the Capitol. Republicans have a sense of fatigue from the past and foreboding for the future, specifically the 1986 midterm elections.

Only four months into his second term, Reagan has been forced to retreat, compromise or stall on almost every major issue: defense spending, domestic budget cuts, the MX missile, aid to the antigovernment rebels in Nicaragua and Social Security cost of living adjustments. Only on taxes, where Reagan has adamantly opposed any increases, has Congress been willing to adopt his position unreservedly.

"There isn't a single legislative battle this year that he hasn't compromised on to avoid losing," said a House Democratic leadership aide. "He doesn't command a mandate among the American people such that he can marshal public opinion to affect Congress . . . . "

At this point in 1981, Reagan was using a united GOP as a battering ram to push his program of domestic retrenchment and military buildup through a vanquished and trembling Democratic opposition. Now it is the Republicans who are in nervous disarray, and the Democrats are showing signs of regaining their footing.

The emergence of a forceful Democratic opposition was dramatized last week in both the House and Senate by two individuals who, out of their sharply contrasting backgrounds, may have brought the party closer to a common ground on critical issues than all its ethereal soul-searching of recent months.

In the House, new Budget Committee Chairman William H. Gray III (D-Pa.), overcoming fears among some lawmakers that a black clergyman from inner-city Philadelphia would have a hard time finding a Democratic consensus, produced a budget that eventually won support even of party conservatives who had once bolted to Reagan on budget issues.

In the Senate, coming from the opposite end of the political spectrum, conservative Sen. Sam Nunn (D-Ga.) capitalized on years of opposition to the MX missile by liberals of both parties to force Reagan to agree to halve his request for 100 of the weapons. Coupled with Senate and House votes to curtail the Pentagon budget, the MX deal in the Senate appeared to signal formation of a new defense coalition dedicated to restraining the Pentagon.

In part, the Democrats' new force in military policy stemmed from Reagan's continued push for ever more defense spending -- an "overrreaching that backfired," according to Sen. Warren B. Rudman (R-N.H.).

But Democrats said it also reflected a gradual synthesis among Democrats, who have been bitterly torn over defense in recent years. Many conservatives who backed Reagan in earlier years abandoned him on the budget this year. "There's much less common ground for us to walk on," said Rep. Marvin Leath (D-Tex.), one of the conservative Democratic "Boll Weevils" who crossed party lines to help Reagan in his first term but came back to the Democratic fold this year.

In the House, Democrats have been curiously emboldened by last year's elections because their losses were not as heavy as expected, and many of the seats they did lose were in southern conservative areas where Democratic incumbents had generally supported Reagan. "It shows you can be against his policies with no retribution; it shows you can not only stand up against Reagan's policies but you're expected to," Coelho said.

Republicans, who lost any chance to play a pivotal role in the House because of their failure to do better in last November's elections, are now increasingly split as different ideological factions fight to control direction of the party in the post-Reagan era. This was apparent during votes last week in the House on various budget proposals when barely half the Republicans supported the official GOP alternative.

"Ronald Reagan is a lame duck, he's not going to be on the ballot again and that makes a big difference," said Rep. Roy Dyson (D-Md.). "My 1984 opponent got nowhere until he just started talking about Ronald Reagan this, Ronald Reagan that. That's when he started picking up. Well, Ronald Reagan's not going to be able to do that again."

In the Senate, except for Nunn's move on defense, it is generally the Republicans who are setting the priorities that have forced Reagan to move away from his original positions on issue after issue this year.

House Republicans have the luxury, and the frustration, of being in the minority, but Senate Republicans control their chamber and enjoy the power control gives. Aware that Democrats could easily topple them in 1986, Senate Republicans feel a pressing need to demonstrate that they can act responsibly and get results. That spirit was evident in their incessant prodding of the White House on deficit reductions.

"One reason the president's popularity is down is that this thing the deficit struggle has drifted on too long. People are asking where are the results," said Sen. Mark Andrews (R-N.D.), who faces reelection next year in a state where high deficits are blamed for interest rates that are hurting farmers.

Sen. Charles E. Grassley (R-Iowa) is almost a metaphor for Republican lawmakers' relations with the Reagan. Elected on the crest of the Reagan tide in 1980, he has made independence of the White House a hallmark of his reelection campaign for next year. White House aides have hinted that Reagan might retaliate by not campaigning for him in Iowa. Grassley has responded with a shrug. "It really isn't that Reagan is weaker, it's that we are stronger . . . . We're stronger than we were in '81," Grassley said.

Said a House Republican aide: "There has been a realization over the last four years that we guys have to get elected alone."