President Reagan is sending a congratulatory message to South Korean President-elect Roh Tae Woo, the White House announced last night, as administration officials expressed guarded optimism that his election victory will lead to greater democracy and stability.

White House spokesman Marlin Fitzwater said Reagan was to send the message to Roh after being briefed on the Korean balloting, in which "Mr. Roh appears to be the winner by a substantial margin."

State Department spokesman Charles E. Redman, in an earlier statement, said the opposition charges of election fraud were "noted," but that the U.S. government has no "independent judgment or corroboration" of them. He urged that the allegations be dealt with "as fairly and quickly as possible so that the process of reconciliation can proceed."

Democrats in Congress who have been on good terms with the South Korean opposition appeared ready to welcome the results in the absence of convincing evidence that of large-scale fraud.

"The Korean people seem to have been too concerned about instability" to elect the opposition, said Sen. Alan Cranston (D-Calif.), chairman of the Senate Foreign Relations subcommittee on Asian and Pacific Affairs. He added, "I believe the fact that the opposition was split did them in."

Rep. Stephen J. Solarz (D-N.Y.), chairman of the comparable House Foreign Affairs subcommittee, said that "fraud on a massive scale that would have affected the outcome so far has not been demonstrated." In the absence of such evidence, Solarz said, "one has to assume it was a relatively fair electoral process reflecting the will of the people."

The main U.S. objectives in the Korean political situation had been described over many months as a credible election, accepted by the Korean people, which would bring about the first peaceful transfer of presidential power in South Korea's 40-year history as a state.

A senior State Department official said that "at this stage" those objectives seem to have been met by Roh's election by a substantial margin over a split opposition. "We look at the large percentage of people voting, more than 90 percent, and the fact that it went off like clockwork, with no violence," said the official, who asked not to be quoted by name. He said no reports had been received from Seoul indicating "large scale fraud."

The important thing now, according to the State Department official, is for Roh to move toward the national reconciliation he promised, to create stability as well as legitimacy for his government. "Based on the steps he took in June and July, one can be encouraged by his flexiblity and hope that he can be very forthcoming now," said this policymaker.