Democratic presidential candidate Edward M. Kennedy greeted the newly chosen Republican presidential nominee with a biting rhetorical blast yesterday, calling Ronald Reagan's tax cut proposal "Laetrile for the American economy" and his social programs "a rendezvous with nostalgia."

But Kennedy, reiterating his own determination to continue his underdog struggle for the Democratic nomination, said that Reagan could win in November if Democrats adopt conservative policies to counter Reagan's proposals.

"We must face the fact," Kennedy said in a lengthy speech before a small audience at American University here, "that the Republicans have emerged from (the Detroit) convention with a long lead in the election. To regain the lead, Democrats . . . must recommit ourselves to serve the disadvantaged and disaffected . . . and to renew the quest for peace."

Kennedy said it did not bother him to hear his own criticisms of President Carter repeated by several speakers at the Republican National Convention this week. "When Republicans invoke my criticism," he said, "they are only capitalizing on a correct assessment of our national crisis."

The Democratic challenger, who has stayed in the race despite Carter's overwhelming lead among Democratic convention delegates, argued that his criticism of Carter's administration does not constitute criticism of Democratic Party principles. The president, he said, has abandoned the party's traditional policies.

For that reason, Kennedy argued, Republican orators in Detroit were wrong to complain about "Democratic policy failures" under Carter's presidency. "Democratic policies have not failed these last four years," Kennedy said. "They have not been tried."

Attacks like that on Carter have been part and parcel of Kennedy's campaign speeches for months. What was new yesterday was the equally harsh attack on Reagan and the Republican Party platform.

Kennedy, who read his speech from a prepared text in dry tones, came to life when he zapped Reagan's proposal for a 30 percent income tax cut.

"Their untargeted tax cut . . . would create more inflation than investment." Kennedy said. "It is Laetrile for the American economy. And it would operate in a wonderfully Republican way -- by giving far more to those who already have the most."