President Reagan has assured Supreme Court nominee Judge Robert H. Bork that he does not intend to abandon the fight for his confirmation, White House officials said yesterday, but undecided and pro-Bork senators expressed skepticism that Reagan can round up the necessary 50 votes.

As he departed for Camp David yesterday afternoon, Reagan told reporters he was not ready to concede Bork's defeat and said he will continue "working as hard as I can to see that he is confirmed, as he should be."

But two undecided Democratic senators who met with Reagan yesterday and then with reporters had no words of encouragement for the White House. Sens. Dennis DeConcini (Ariz.) and J. James Exon (Neb.) said they told the president the nomination is in serious difficulty.

"I think Judge Bork is in big, big trouble," said Exon. "I got the impression that unless they could get 49 or 50 senators nailed down in the next few days the nomination would be withdrawn."

Even Sen. Richard G. Lugar (R-Ind.), a Bork supporter who attended a Rose Garden ceremony where Reagan appealed for Bork's confirmation, said it was "not apparent" where the president could get the needed votes.

On Capitol Hill, Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) became the fourth southern Democrat to declare he will vote against Bork's confirmation. Because of his stature and seniority, Bentsen's opposition was seen as another heavy blow to Bork's chances of confirmation.

Bentsen was one of nine Democrats who announced they will oppose Bork. The others were Sens. Jeff Bingaman (N.M.), John F. Kerry (Mass.), Max Baucus (Mont.), Barbara A. Mikulski (Md.), Carl Levin (Mich.), Donald W. Riegle Jr. (Mich.), John D. Rockefeller IV (W.Va.) and Timothy E. Wirth (Colo.).

Bork's supporters countered with formal endorsements by Sens. David L. Boren (D-Okla.), John McCain (R-Ariz.) and Phil Gramm (R-Tex.). The decision by Boren, a Democrat from the Southwest, was discounted by Bork's opponents, who said he was never considered a swing vote.

An Associated Press tally of senators who have declared how they will vote showed 32 senators for Bork and 26 against him. But a private White House count showed that Bork would have to win the support of nine or 10 of 17 "actually undecided" senators, and one administration source acknowledged "that this is a very tall order."

"Judge Bork is clearly defeated," said Sen. Bob Packwood (R-Ore.), adding that he and three other Republicans were "irrevocably committed" to opposing Bork and that two other GOP senators may vote against confirmation.

However, White House officials said that Reagan pledged, in private as well as public, that he would not give up the fight. They said Reagan refused to even discuss alternative appointments and had told Bork, in a meeting at the White House on Thursday, that he was "with him all the way."

White House chief of staff Howard H. Baker Jr. told reporters he had called 45 senators within the past few days. "It's a tough deal, but it's still winnable," Baker said.

Later, on the "MacNeil-Lehrer News Hour," Baker said, "There is still a realistic prospect of success. There is no prospect of withdrawing the nomination. There has not been an avalanche of defection, and I don't think there will be."

Similar declarations were made by other White House officials as they tried to counter the growing impression that Bork's chances of winning confirmation may be irretrievably lost. White House legislative liaison Will Ball said the nomination "can still be won because there are enough undecided senators out there to put us over the top." Spokesman Marlin Fitzwater said the president "has a very ambitious program right to the end to try to turn people around."

Reagan will make another appeal for Bork today in his weekly radio address. Fitzwater said he also will be calling senators during the weekend from Camp David. More meetings with undecided senators have been scheduled for next week.

But one White House source, while saying that this intense effort was "likely to close the gap," acknowledged that it may not be enough to push Bork to the 50-vote level, at which Vice President Bush could break a tie in his favor.

The source said that if Bork's defeat is apparent after the Senate Judiciary Committee votes on the nomination on Tuesday, Reagan would face "the dilemma" of continuing to fight for a "lost cause" or accepting defeat and quickly submitting another nominee.

"The problem if we reach that point will be one of timing," the source said. "If Bork can't win, it will be important to move quickly and submit another nominee so that opponents can't delay selection of a new justice until next year."

DeConcini said after meeting with the president that he had warned Reagan that Bork is "in deep trouble" and that momentum is working against the White House. The Arizona senator, a key undecided member of the Judiciary Committee, said he would announce his vote Monday.

On Thursday, the Bork nomination was damaged when Sen. Arlen Specter (R-Pa.), also a member of the committee, announced opposition to Bork. Reagan met yesterday with Sen. John Heinz (R-Pa.) in an effort to win his vote, but Heinz said afterward that he remains undecided.

Bentsen described as "repugnant" Bork's statement in the 1960s that the public accommodations section of the Civil Rights Act of 1964 was based on a principle of " 'unsurpassed ugliness.' " Bentsen said that "America has set its house in order when it comes to civil rights," and added, "I question whether very many Americans -- black, white, Hispanic and others -- want to turn back the clock and revisit those questions."

Bentsen also used his announcement to urge that Appeals Court Judge Patrick E. Higginbotham of Dallas, already mentioned as a possibility by administration sources, be nominated should Bork be defeated or his name withdrawn.Staff writer Edward Walsh contributed to this report.