The greatest threat to President Carter's reelection may not necessarily come from his eventual Republican opponent, but from within the ranks of his own party.

An early assessment of nationside Democratic sentiment and the 1980 election indicates it is not Carter but Sen. Edward M. Kennedy who is the first choice of party members to be the 1980 standard-bearer. Kennedy is not only the first choice of Democrats from a list of six possible Democratic nominees in 1980, but the Massachusetts senator holds an even more impressive lead when up against Carter alone.

When asked to name their first choice for the 1980 nomination from a list of six prominent Democrats, 36 percent selected Kennedy while 29 percent picked Carter, California Gov. Edmund G. (Jerry) Brown Jr. was third on the list with 12 percent of the vote, followed by Vice President Mondale (8 percent), Sen. Henry M. Jackson of Washington (5 percent) and Rep. Morris K. Udall of Arizona (3 percent).

When Democrats in the March 31-April 3 survey were asked whom they would prefer if the 1980 convention narrowed to a chioce between Kennedy and Carter, Kennedy won the showdown by a healthy 53-to-40 percent.

The survey was asked:

Here is a list of people who have been mentioned as possible presidential candidates for the Democratic party in 1980. (Respondents were handed a card with six names listed). Which one would you like to see nominated as the democratic candidate for president in 1980?" (TABLE) Kennedy(COLUMN)53 Pct. Edward Kennedy(COLUMN)36 Pct. Jimmy Carter(COLUMN)29 Edmund Brown Jr.(COLUMN)29 Walter Mondale(COLUMN) 8 Henry Jackson(COLUMN) 5 Morris Udall(COLUMN) 3(END TABLE)

Only once in the 43-year history of the Gallup Poll has an incumbent president eligible for election been defeated in a showdown test of strenth. .. .. ..

In September 1967 Kennedy's brother Robert was the choice of 51 percent of Democrats for the 1968 nomination while 39 percent preferred then-President Johnson.

Also attesting to Ted Kennedy's strength are the findings by region. In Carter's native South and his stronghold in the 1976 election, Kennedy holds the president to a stand off, winning 46 percent of the vote compared to 44 percent for Carter. Outside the South, however, Kennedy is a clear 56-to-38 percent winner.

That Kennedy should make such a strong showing, even against an incumbent president of his own party, is not a total surprise. He has long been a favorite of Democratic voters, and was their top choice in the poll even though he has said he would not be a candidate.

Because of recurring speculation that Brown will run for the nomination if he succeeds in winning reelection as governor of California, the survey also sought to gauge his current strength against the two Democratic frontrunners.

Results show Brown losing to both Kennedy and Carter is head-to-head constests. Kennedy defeats Brown by 64 to 27 percent.