Conservatives began a major letter writing and advertising campaign yesterday designed to block congressional approval of the administration's new Panama Canal treaties.

The campaign began on a day when Sens. Strom Thurmond (R-S.C.) and Jesse A. Helms (R-N.C.), who last week visited the Panama Canal Zone, went on national television to denounce the treaty proposals as harmful to American economic and defense interests and as a capitulation to "blackmail."

Helms, who appeared on "Meet the Press" (NBC, WRC) with Thurmond, also promised there would be a Senate filibuster against ratification of the treaties. "You can count on it," he said.

Carter administration strategists believe they now have about 50 of the 67 votes needed in the Senate to ratify the treaties, but that they will have to go through a gruelling national debate to win the other 17.

An Associated Press poll released yesterday, coupled with the remarks by Helms and Thurmond and the beginning of the conservaties campaign, indicates that administration's assessment of its chances might be too optimistic.

The AP poll of the Senate showed 17 members who say they will support the treaties drafted Aug. 10 by negotiators for the United States and Panama, 17 opposed, and 62 members who said they were undecided or unwilling to commit themselves. The other four senators could not be reached for comment, the wire service said.

But even if the administration is correct in its present count of pro-treaty Senate votes, there is little likelihood the treaties could be retified this year.

The wire service quoted Sen. John Sparkman (D-Ala), chairman of the Senate Foreign Relations Committee, as saying there was "no reasonable chance" the matter could come to the floor before Congress' scheduled October adjournment. Sparkman's committee must first approve the proposals.

Sparkman reportedly is "open-minded" about the treaties. But groups like the American Conservative Union which launched its newspaper advertising campaign yesterday, are hoping to close Sparkman's mind on the issue soon.

"In our ad campaigns, we are concentrating on he South," said ACU spokesman Fran Griffin. "As far as the treaties go, if the Southern Democrats remain with us on this issue, we could defeat it in the Senate," she said.

Griffin said her organization plans to spend $50,000 his week on newspaper advertisements, mostly in Southern states. Money to pay for the ads came from an ACU letter appeal conducted in May, she said. She said nearly all of those funds will be spent this week, but that the ACU is counting on the ads to generate more funds for opposiing the new treaty proposals.

An ACU newspaper as appearing yesterday in Jackson, Miss, bannered the conservative position on the Panama Canal issue:

"There is no Panama Canal.There is an American Canal at Panama. Don't let President Carter give it away," thead read.

The ad warned that America's international image would be hurt if it went through with the agreements that would grant Panama control over the Panama Canal and the Canal Zone by the year 2000, but would also give the United States the right to defend the waterway indefinitely.

The ad also appealed to citizens to pressure their senators - through phone calls and letters - to vote against ratifications.

Griffin said the ACU advertising effort "is sort of an exploratory campaign" that may be scrapped, modified or expanded, depending on the results. But she said her organization is optimistic that this time "the conservatives are on the right side . . . [because] polls show a majority of the people agree with us."

Sen. Adlai Stevenson (D-III.), an acknowledged backer of the new treaties, might agree with that. According to the AP survey, the senator's office, since the first of the year, has received 5,600 letters opposing a new Panama accord and only five in favor of such an agreement.

That direct mail campaign is expected to increase under the guidance of Richard Viguerie of Falls Church, who has used the mails to raise millions of dollars for conservative causes.

Viguerie was quoted yesterday by his United Press Internatiional as saying his direct mail operation will send out "a minimum of several million letters" to raise "grass-roots" opposition to the new Panama treaties.

"We are organizing a massive campaign to mobilize the 70 to 75 per cent of the American people who support our position and make that view known to Congress," he said.

Both Griffin and Vaiguerie said they have sought and are receiving "some support" from labor union officials in the effort to defeat ratification of the treaties.

Thurmond, who also serves as chairman of ACU's Panama Canal Task Force, said yesterday the fight against the treaties is necessary because the canal is "American property" and, "I don't see how we can let American property - or the rights of Americans - go down the drain."

-Would you go to war to keep it?." Thurmond was asked in the television interview.

"I think we cannot afford to let other people take our property, and I think we'd have to take whatever reasonable steps are necessary to protect the property of the people of this country," Thurmond said.