The Senate yesterday opened its much-delayed debate on President Reagan's nomination of Kenneth L. Adelman to head the Arms Control and Disarmament Agency, focusing less on Adelman than on White House arms control policies.

All sides agreed that Thursday's vote will be very close, hinging on whether the White House can talk a few Republicans out of their misgivings over Adelman's qualifications for, and interest in, serious arms bargaining with the Soviet Union.

Assistant Majority Leader Ted Stevens (R-Alaska) predicted that Adelman, 36, now deputy U.S. ambassador to the United Nations, would be confirmed in the arms control post by one or two votes at most. Sen. Paul E. Tsongas (D-Mass.), a leading Adelman critic, said the balance "keeps shifting every day" under "a full-court press" from the White House.

Tsongas told reporters that the critics were "able to raise the issue of arms control, using the nominee as a process," and that, even if Adelman is confirmed, "we have accomplished our purpose" to that extent.

"Most members of the Senate would rather this nomination just go away," he said.

Sen. Charles H. Percy (R-Ill.), chairman of the Foreign Relations Committee, which in February rejected Adelman 9 to 8 but then agreed to let the full Senate decide, set the tone of Adelman's defense by saying the prolonged controversy meant that Adelman "will bear the burden of proof that he actually is a strong advocate of arms control."

In its zeal to have him, the administration has "agreed to some very important undertakings and strong commitments," Percy said, including personal involvement in arms issues by Secretary of State George P. Shultz and beefed-up staffing for the floundering agency. "If we don't confirm Mr. Adelman, they are relieved of the responsibility of these strong commitments," he said.

Leading the opposition, Sen. Claiborne Pell (D-R.I.) responded that Adelman "has been given a fair test and has failed that test." In Adelman's three confirmation hearings before the Foreign Relations Committee he "failed to demonstrate an understanding of arms control issues," Pell said, and showed in his writings "a greater concern with the politics of arms control than with its substance."

"He has much too limited a view of what his own role would be if confirmed, indicating he would be a 'contact point' rather than a 'focal point' for negotiations," Pell continued. But when the agency was set up 22 years ago, "The last thing that we wanted was for ACDA to be an echo chamber for policies developed in the Pentagon."

Sen. Jesse Helms (R-N.C.) called those arguments "a charade" focusing on whether Adelman "misled his critics down a maze of their own construction."

A test of sentiment is possible today over an expected move to kill the nomination by sending it back to the Foreign Relations Committee for reconsideration. Top leadership aides expressed confidence that such a move would be defeated, and Tsongas said it was not certain to be offered.

Tsongas said Vice President Bush had been telephoning undecided senators, and Adelman was spotted visiting some of them as the debate continued. "I'm just talking to those who have made requests" for a meeting, he told the Associated Press.

An aide to Sen. Mark O. Hatfield (R-Ore.), one of the key undecided votes, said Hatfield "generally believes the president should get who he wants," but that "he's concerned that Adelman seems more committed to the area of arms buildup than arms control."