The youngest son of America's best known political clan turns 48 on Friday, and in the family tradition Edward M. Kennedy has made his birthday a political event.

Kennedy has build much of his presidential campaign for the past week around a series of fund-raising birthday parties -- crowded, smoke-filled affairs that end with the candidate trying to blow out a big spread of candles top an enormous cake.

Blessed with a politician's lung power, Kennedy usually gets them all out. Then he announces that this means he will get his wish: a victory in next Tuesday's New Hampshire primary.

But when the candidate departs and his motorcade sets off for the evening's next party, Kennedy's campaign aides settle into the bus and admit glumly that the birthday wish is unlikely to come true. The Kennedy people are talking now as if the only question in New Hampshire is how big Jimmy Carter's victory margin will be.

The campaign's official answer -- which is "leaked" to anyone who bothers to ask -- is that Kennedy's own pollster, Peter Hart, is projecting a 2-to-1 Carter win. But that answer raises a second question -- if Kennedy loses that badly in his native region, can he mount a realistic challenge to Carter anywhere else?

Kennedy, at least, says he can. The Massachusetts senator said this week that he will have enough money and enough supporters to keep going, no matter what happens in New Hampshire "You guys will see," he told reporters. "We'll take this right down to the convention."

The senator is already looking beyond New Hampshire for more fertile political territory. He has interrupted the campaigning in New Hampshire every day this week for fund-raising forays into his home state.

Today he spent the entire day campaigning in Massachusetts. Friday afternoon he will be back in New Hampshire for one more birthday party and several other stops. Saturday he leaves for speeches in Kansas and Oklahoma and Sunday he will be fund-raising in New York City.

The Kennedy staff's fatalism about New Hampshire disappears when the subject changes to the Massachusetts primary one week later. The senator's home state will send 111 delegates to the Democratic National Convention, six times as many as New Hampshire. This spurs Kennedy's delegate-counters to numerical extrapolations that suggest their man will be ahead of Carter in delegate strength the day after Massachusetts votes.

Kennedy's optimism here is based on polls that show him leading Carter by a substantial margin in Massachusetts and on his own evident belief that home-state Democrats will not let him down. But Carter has been working in Massachusetts. The president has the support of Gov. Edward M. King and yesterday the Democratic leadership of the state legislature also endorsed Carter.

Boston politicians debated hotly today whether the legislative endorsement means that the president might do well here, but no consensus emerged.

The numbers look bad for Kennedy in the southern primaries March 11, one week after Massachusetts. Florida, Alabama and Georgia will elect a total of 208 Democratic delegates and every one expects Carter to snare a big majority of them.

Further, whatever optimism there was in the senator's camp seems to have been dissipated by surveys reported in Time magazine and The New York Times this week, which showed Kennedy losing ground to Carter nationwide after the Georgetown University speech that he hoped would revive his effort.

the Kennedy campaign held 60 birthday party fund-raisers all over Massachusetts tonight, with dozens of nieces and nephews deputized to provide a Kennedy presence at each.

The candidate, with his 89-year old mother, Rose, his wife, Joan, and two of their three children, blew out the candles (on a White House-shaped cake) and made his wish at a $250-per plate dinner at the Harvard Club here.

Joan stepped to the microphone to offer a toast to her husband, then paused. "I just got my cue cards all filled with icing," she said.

With the icing cleared, she said, "I asked Ted how it feels to be almost 48. He said he thinks he's really still down in the 30s somewhere, but that he hopes the undecided will break in his favor."

Kennedy stood, gave his wife a peck on the cheek, and said. "I think everybody understands who the best campaigner of 1980 has been in our family, and that's Joan." The 325 diners responded with prolonged applause.

In other developments today:

First Lady Rosalynn Carter arrived in Manchester to begin two days of campaigning for the president with the warning that the outcome will "probably be very close" despite pools showing Carter far ahead of Kennedy.

Gov. Edmund G. (Jerry) Brown Jr. challenged Kennedy to debate him on national energy policy before Tuesday's New Hampshire primary. Brown charged that Kennedy is using "deceptive ambiguities" in stating his position on nuclear energy, "attempting to create a false impression that he flatly opposes further licensing of nuclear power plants. He is creating a credibility gap that will haunt his campaign in the days ahead."