Majority Leader Robert C. Byrd (D-W. Va.) said yesterday that the Senate would defeat the new Panama Canal treaties if forced to vote on them this year.

To bring them up this fall . . . would ensure . . . rejection," Byrd told reporters at his regular weekly news conference.

" . . . Time is required to inform the American people, because as long as the polls are extremely negative, it would be just as difficult to get two-thirds of the Senate to go along." He said polls have shown 75 to 80 per cent of the public disapproves of the treaties.

Some senators have been expressing sentiments similar to Byrd's for weeks. But yesterday was the first time the majority leader said publicly that the necessary 67 votes for approval are not there.

Sol M. Linowitz, who helped negotiate the treaties, said two weeks ago that the Carter administration would decide around this time whether it had gained enough Senate support to push for a Senate vote this fall.

Linowitz said the administration wanted a vote before Congress adjourns for the year - now predicted to be some time in late October or early November - "if the votes are there."

Byrd said yesterday he has told President Carter of his feelings. Two weeks ago the majority leader said, "anyone who thinks I will bring it up before January is living in a dream world," because neither the senators, nor the public, had had enough time to study the terms.

He expanded on that statement yesterday, saying he plans to call the treaties up for a vote early next year. Polls have "indicated a shift in public opinion already, which would indicate that the people are becoming better informed with the passage of time . . . " he said.

Byrd said his won mail is running "about 4,000 to 6" against approval, but that much of the opposition appears to be an orchestrated letter-writing campaign.

The Senate has before it two treaties considered as one. One gives control of the canal to Panama by the year 2000. The other gives the United States the right intervene militarily to maintain the neutrality of the canal zone.

Hearings begin this week in House and Senate committees on the treaties, and Byrd said they would give the administration a good chance to try to sway public opinion.

The White House has done "very well" in arguing its case so far, he said. "The President has met with public opinion makers for months from various states to keep them informed."

There is "no chance" of getting the required 67 votes during the next few months, Byrd said. He expects lengthy floor debate, but "I would hope the treaties would be disposed of one way or another by the early part of the year."