Washington — Ronald Reagan will be after the ''baby boomers.'' One lesson the Republicans learned from the Democratic primaries, Reagan campaign strategists say, is that many of those young people who have supported Gary Hart indicate they will not support Walter Mondale if he is the presidential nominee. The President will therefore court this constituency, among others, when he gets out on the campaign trail.
''Hart dramatized the existence of a youthful constituency, and we will pay attention to them,'' says Charles Black, a key Reagan campaign consultant. ''Exit polls showed that many liked Hart but will switch to Reagan if Mondale is nominated.''
Reagan campaign officials are careful not to overestimate the potential votes among the 18- to 39-year-olds in the baby-boom generation. Young people's voting record is worse than the national average. But GOP officials are conscious of a growing potential in this age group.
''There is a demographic change going on when as many as 100 million baby-boomers aged 18 to 39 are eligible to vote and only 70 million Americans older than 39 are eligible to vote,'' says an official at the Reagan-Bush campaign head-
In the aftermath of the Democratic primaries, Reagan operatives make some other observations that have implications for the Reagan campaign:
* There is not likely to be a brokered Democratic convention. Former Vice-President Mondale will have ''a comfortable working majority'' and, although he will have to accommodate the other candidate groups, he has the votes to win.
* Mondale managed to capture more than 50 percent of the vote in only one state, West Virginia, indicating he does not now have a strong hold on the Democratic constituency. This gives the Reaganites ''tremendous potential.''
* Current polls indicate Mr. Reagan would still beat Mondale or Senator Hart if the election were held today. Especially encouraging to the Republicans is a USA Today poll published this week showing Reagan with an almost 40-point lead over Mondale among independents. The President, therefore, is seen to have good possibilities for ''getting half of the independent vote he must get'' to win in November.
Despite the current optimism, Reagan reelection planners continue to anticipate a close race in November and are cautioning their troops against complacency. Mondale is viewed as not having managed to sustain his momentum with each primary victory and as not having captured a large margin in most states. He failed to wrap up his nomination by mid-March, as originally planned.
But several factors will operate to narrow the Republican-Democratic gap as the election draws near, GOP operatives say: (1) historically, electoral landslides are generally followed by close elections; (2) Mondale will have a built-in advantage if he holds on to his Democratic coalition, because Democratic voters outnumber Republicans; and (3) a challenger always chips away at an incumbent.
''I don't think (the long-drawn-out Democratic race) means a whole lot,'' Mr. Black says. ''The Democratic Party has a lot of voters and resources. And once they put their coalition together Mondale will be strong.''
It is one of the ironies of the campaign that President Reagan, who himself is older than 70, has higher approval ratings among younger than older voters. A recent Los Angeles Times poll showed Reagan winning over Mondale and Hart among both groups but doing best among the 18- to 39-year-olds. The reason for this, GOP operatives say, is the President's early call for social security reform, an acknowledged ''gaffe'' still being effectively exploited by the Democrats.
''This is the only segment of the population where there has been an erosion from the 1980 base,'' says John Buckley, deputy press secretary of the Reagan-Bush campaign committee.
Reagan will try to repair that erosion. But he will also target the young independents and Democrats who are Senator Hart's primary source of support. Campaign officials say the Republicans can focus on the President's ''strong leadership'' and the invigorated economy in their appeal to the baby-boomers.
''When Hart made his big surge and the press focused on the 'candidate of the future,' we realized that we have to point out that four years ago it was the Republicans who were the party of the future and how Reagan embodied that spirit of change,'' Mr. Buckley says. ''Reagan in 1980 represented a strong break with the past and, as President, he has moved the country in a new direction.''
Mondale's ties with special-interest groups are also seen as giving Reagan ammunition to use among the independents and young. ''He can point out that he is independent of these interests,'' Buckley says.
To benefit substantially from the baby-boomers, Reagan would have to reverse voter patterns in recent elections. Fewer than half of those born since 1946 voted in the 1972, 1976, and 1980 presidential elections, according to the National Journal. The voting numbers are gradually rising but are still short of the national average. In 1982 only 35 percent of eligible voters between the ages of 20 and 38 turned out to vote; the national turnout was 48 percent.
''The younger boomers (born between 1946 and '54) are still the most unsettled group in the electorate and the least likely to vote,'' the magazine says. But studies indicate that ''they are much more conservative than slightly older college students.''
The National Journal quotes Lee Atwater, deputy director of the Reagan-Bush committee, as saying the baby-boomers are ''libertarians'' - economically conservative and socially liberal. That means, according to Mr. Atwater, highlighting Reagan's economic record but not his record on social issues.