President Reagan, who chose his legislative targets carefully in launching his first term, has begun this year with a string of surprisingly difficult fights with Congress that have eroded his political capital on the eve of his campaign for tax simplification.

Despite his 49-state reelection victory and continuing high approval ratings from the public, Reagan is struggling to avoid further setbacks in Congress of the kind that have troubled the White House since he promised a "Second American Revolution" in his Inaugural Address in January.

The farm-credit crisis, struggles over the MX missile and aid to Nicaraguan rebels, controversy over the Bitburg cemetery visit and an ambitious budget proposal that target middle-class programs while continuing the defense buildup have sapped Reagan's strength on Capitol Hill, according to lawmakers, White House officials and other political analysts.

Even as he prepares to launch a tax-simplification initiative that may become a centerpiece of his second term, Reagan has been beset this week by problems. The administration is fighting in the Republican-controlled Senate against capping the MX missile force and is facing certain reversal of its budget priorities in the Democratic-controlled House.

Earlier, in a compromise with the Senate, Reagan was forced to accept significant changes in his budget priorities.

Four years ago, it was much different. Then Reagan was successful because he was able to focus the congressional agenda almost entirely on his economic program, thereby avoiding the kind of distractions that have cropped up repeatedly this year.

"This is year five, and not year one," said Craig L. Fuller, Vice President Bush's chief of staff and a key White House official in the first term. "In the fifth year of what is an eight-year presidency, you have to start with the agenda you're given. You don't have the luxury of beginning with a blank piece of paper."

For all his troubles, Reagan still holds power on Capitol Hill. He will try to improve the prospects for tax revision with a series of speeches starting Tuesday. He is expected to win approval of aid to the Nicaraguan rebels and may put a dent in the deficit before this session of Congress is over.

But the White House effort this year has not been as carefully plotted as it was in 1981, when Reagan was given a study by several top political strategists, including pollster Richard B. Wirthlin, dubbed the "Initial Actions Plan" -- a blueprint for his early congressional victories.

The plan, delivered to Reagan on Jan. 29, 1981, warned the new president that when Jimmy Carter had assumed the presidency, he tried to move on too many fronts at once -- energy, welfare revision and taxes, among others -- leaving Congress uncertain of his priorities.

The study advised Reagan to focus on his economic program in the first half of 1981, saying the public would approve of an aggressive effort if it stressed the then-critical inflation problem.

The White House deftly controlled the agenda, in part by deferring action on conservative social issues such as abortion and by postponing difficult foreign policy decisions. The major unexpected event, the March 30, 1981, attempt on Reagan's life, paid political dividends. It helped Reagan focus on his economic program during his recovery, and it built sympathy for him in Congress.

No comparable plan was presented to the president for the second term, White House officials said. When Donald T. Regan took over as White House chief of staff from James A. Baker III on Feb. 4, he had little time to look ahead. Several officials said the job swap should have been carried out after the November election instead of after the inauguration.

Modern presidents have usually begun their terms with a bank of good will in Congress that is then gradually expended. Baker, in the first term, treated every issue as a campaign event, trying to maximize Reagan's reach while limiting any damage.

Regan, who entered the White House with less political experience than Baker, announced that he would rely as much as possible on the president's communicative skills and judgment as each issue came up.

Even before Congress began voting on his budget this year, Reagan had to use some of his political capital on two issues left over from 1984, the MX missile and aid to the Nicaraguan contras, or counterrevolutionaries. The timetable for both was set by legislation last autumn.

The new White House made no effort to defer these battles.

A third issue -- the farm-credit crunch -- aggravated the crowded agenda as well as relations with Congress.

"When I saw Ronald Regan give a televised address on the budget the same night the House was voting on aid to the contras and right before he took off for the summit, I said, here is the cardinal sin of politics," said Norman Ornstein, a political scientist at the American Enterprise Institute. "You do not let your own momentum trip all over itself. That was Jimmy Carter's classic failing. He never understood that what you do today affects what you do tomorrow."

Sen. Paul Laxalt (R-Nev.) said he thinks that Reagan has done well the first few months "considering, historically, the difficulties most presidents have had in their second four years." But he said "if there was a tactical error" it was "throwing the contras into the middle of it all."

Beyond the crowded agenda, other factors have frustrated Reagan. In the budget debate, for example, the White House sought to eliminate or trim many domestic programs that Congress had refused to tackle in the first term.

"The easy victories are gone on the budget," said Frank Donatelli, a longtime Reagan campaign operative who recently left the White House public liaison staff. He also pointed out that Reagan was seeking a major tax cut in 1981 while his new tax program is "revenue neutral," raising the same revenue as current law but shifting tax burdens.

While Reagan had the allegience of virtually all Republicans in 1981 and went on to build coalitions with conservative Democrats, his second term began with Republicans openly seeking more independence and Democrats digging in their heels in the wake of their stinging 1984 defeat.

"Post-election, the political scars are so deep," Laxalt said, with Democrats wanting to "teach Reagan a lesson."

"The Democrats, at least on the Senate side, have done a marvelously good job of mounting a loyal opposition," he added. "The aura of the original mandate wears off. The magic of a presidential telephone call is still strong, but not electric."