Washington — ''It's not going to be a cakewalk.'' So commented a high Republican official as 1,000 campaign supporters whooped and hollered at the Mayflower Hotel after Ronald Reagan's announcement that he would seek a second term.
Cakewalk or not, the President officially enters the political race with widely acknowledged strengths:
* An incumbent has the advantage of the news media and other resources of his powerful office - resources that Mr. Reagan has used with great skill.
* Reagan's popularity remains relatively high. For a president entering his fourth year in office, some polls show him with the highest approval rating of any president since the '50s.
* The Democrats have yet to coalesce and to forge a political platform and strategy to challenge the President's record.
It is only a matter of time before the tangled Democratic nomination race is resolved, of course. Once the party has made its choice, the character of the campaign will change. Given the strength of Walter Mondale's organization, political observers say, it is even possible the Minnesotan will emerge as Reagan's opponent well before the Democratic convention in July.
Meanwhile, Reagan will ''stay above the fray,'' as his strategists put it, and dominate the limelight. In the weeks and months ahead his political advisers will make sure that he is seen ''acting presidential.'' A carefully crafted, well-timed schedule of public appearances and travels - including a journey to the People's Republic of China in April and a trip to England for an economic conference in June, which will include a sentimental visit to Ireland - will afford many opportunities for colorful media coverage and enable the President to command center stage.
Political experts note that, the defeat of Jimmy Carter notwithstanding, incumbency is a formidable obstacle for a challenger to overcome. ''Beginning in 1900, where sitting presidents have run, they have won reelection 11 out of 15 times,'' says political scientist James Sundquist of the Brookings Institution. ''So unless there's a reason to throw him out, the tendency is to give an incumbent another term.''
For the moment GOP strategists are quietly cheered by the polls. According to the latest Newsweek survey, 52 percent of those questioned said they were likely to vote for Reagan if he ran again. A recent Gallup poll gives Reagan an approval rating of 54 percent and disapproval rating of 37 percent.
Significantly, the high ratings are related more to Reagan's personality and qualities of leadership than to policies and issues. When polls narrow to specific questions about peace, Lebanon, budget deficits, and fairness to the poor, the President fares less well. One recent poll found Reagan leading Mondale by only 3 points.
Assessing the President, political analysts and presidential scholars of various philosophical persuasions seem agreed that he is a strong leader. He conveys a sense of direction, a positive, can-do spirit, a capacity for flexibility without forsaking his ideology.
It is felt that Americans respond favorably to his sincerity, his genial character, his unalloyed optimism, his unabashed patriotism, and his consistency. Mr. Carter's downfall in 1980 is attributed in large part to the perceived absence of these qualities, or at any rate an inability to convey them.
At the same time the Reagan presidency has been marked by many paradoxes, analysts say. Preaching a balanced budget, for instance, Reagan has presided over the largest budget deficit in the nation's history. Promising to strip down big government, he has increased federal spending to where it now accounts for a high 25 percent of the gross national product. Conveying an America that is today stronger and more activist abroad, he has not chalked up any specific diplomatic accomplishments.
As they plot campaign tactics and strategy, the Reaganites are taking serious account of the President's vulnerabilities as reflected in opinion polls. Americans are jittery about the American marines in Lebanon, about chilly relations with Moscow, and about the lack of progress on curbing the runaway arms race. Many segments of the population also are concerned about fairness - the President's tilt toward the well-to-do - and about perceived insensitivity to women's issues and to blacks and other minorities.
This is why GOP operatives are careful to keep their optimism in check. ''We've got tremendous leads,'' says Reagan campaign manager Edward J. Rollins. ''We understand this is not going to be the case nine months from now.''
The Democrats have a credible chance to unseat Reagan, in the view of some presidential experts. It is pointed out that Reagan, while he was elected an electoral landslide, won only 51 percent of the popular vote. The 1980 election was more a vote against Carter than a vote for Reagan. Three years later, Reagan's approval rating is up, say analysts, but not by a huge margin. The question is whether the Democrats, who are the majority party and who turned away from Carter in 1980, are still willing to support Reagan. In 1982, midway into the Reagan presidency, 53 percent of the voters voted for Democrats in congressional elections.
Also, there has been no movement into Republican ranks since Reagan became President, analysts say. The Democrats still have the edge in party affiliation: 40 percent of the registered voters, as against 24 percent for the Republicans. This year, moreover, blacks are being registered as Democrats in large numbers.
It remains to be seen whether various groups who are displeased with certain of Reagan's policies - women, environmentalists, blacks, blue-collar workers - will return to their traditional political home in November, analysts say. Or whether the President is linked solidly enough with a reviving economy and a more self-confident mood in the country to put him over the top once again. It is recognized that it is often the intangible qualities of leadership rather than issues that make the difference when the individual enters the polling booth.
For his part, Reagan sounded the campaign theme of ''America is back'' and struck a lofty presidential posture of bipartisanship even before his official five-minute announcement to run again. As the campaign unfolds, he is expected to continue delivering a message of goals accomplished, hope for the future, and a need for another term to carry out his agenda.
''Our work is not finished,'' he said from his Oval Office Sunday night. ''We have more to do in creating jobs, achieving control over government spending, returning more autonomy to the states, keeping peace in a more settled world, and seeing if we can't find room in our schools for God.''
In addition to the appeal of better economic times, Republican partisans hope that the prospect of a stable, two-term American presidency - after the traumas of Vietnam and Watergate and a succession of one-term or short-term presidents - will figure heavily in voters' thinking. A Republican button manufacturer expressed it this way on one of his creations: ''Go the Distance!''