In a critical test of President Carter's prestige and political influence, the Senate will vote this afternoon on the first of two controversial Panama Canal treaties.

Last night, White House lobbyists and the leaders of both parties in the Senate thought they could count the 67 votes that will be needed to approve the canal treaty if, as expected, all 100 senators vote today. [Text of the treaty appears on Page A18.].

After a reassuring statement of support from Sen. Edward W. Brooke (R-Mass.), who had been wavering, treaty, supporters could count 65 certain votes for approval, and they hinted that they had private assurances from at least two of the last six undecided senators that would put them over the top.

In an ambivalent speech on the Senate floor, Brooke noted that the Carter administration and other treaty supporters had failed to build a national consensus for the treaties - an important reason no momentum developed behind the pacts during nearly a month of Senate debate.

Today's vote is scheduled to take place at 4 p.m. and will be covered live by channels 4 and 9 and WETA radio. The final tally is expected to show that Senate sentiment has hardly changed since the debate began.

Treaty opponents privately acknowledged yesterday that the odds favored Senate approval of the treaty, though they would not concede defeat.

Supporters and opponents agree that this will be one of the those rare votes that genuinely deserve the much-loved Senate label "historic." The outcome could have great importance not only for Carter's presidency, but for the future of the Republican party as well.

Carter has made this vote a critical test for his presidency. In recent days the administration's highest officials have urged undecided senators to vote yes to prevent a crippling blow to the president's authority to conduct foreign relations.

Having made this argument, administration officials concede, defeat now really could be crippling.

But senators and government officials say it is not easy to predict the consequences of either victory or defeat for Carter in today's vote. For one thing, victory represents only half of the Panama Canal loaf - a second treaty remains to be considered, and approval of the first does not guarantee approval of the second.

Brooke said yesterday he would not vote for the second treaty, covering details of the turnover of the canal to Panama, unless it is changed, despite his decision to vote today for the so-called "neutrality treaty."

Some opponents, notably Sen. Richard Schweiker (R-Pa.), have argued that defeat would not be cataclysmic, because Carter and the government of Panama could quickly negotiate modest changes in the neutrality treaty that would make it acceptable to the one or two additional senators needed to win two-thirds approval.

But administration officials doubt this would be so easy. They fear that the president would have to immediately reinforce the U.S. garrison in the Panama Canal Zone if the treaty is rejected today, as a signal to the Panamanians that the United States would not tolerate any new interference with the zone. But Panamanians could easily react to this as a provocation, with unforseeable consequences, these officials argue.

Carter offered a fresh challenge to Panamanian sensibilities yesterday when he accepted a proposed reservation to the neutriality treaty offered by Sen. Dennis DeConcini (D-Ariz.). Abandoning earlier policy, the president said he could accept the reservation, which would give the United States the right to use its armed forces unilaterally to intervene in Panama under certain conditions.

DeConcini's reservation says that notwithstanding any provision of the neutrality treaty. "If the canal is closed, or its operations are interfered with, the United States and the Republic of Panama shall each independently have the right to take such steps as it deems necessary, including the use of military force in Panama, to reopen the canal or restore operations . . ."

For the record, administration officials say this merely spells out rights already granted in the treaty itself, but in fact it has a political significance in Panama that goes far beyond the treaty.

Until now Panama's leader, Gen. Omar Torrijos, could argue that the deliberately vague language of the treaty only provided for joint Panamanian-American action to defend the canal's neutrality against an external threat. The United States did not accept that interpretation, but the treaty did not contradict it either.

DeConcini's reservation would stay out of Panamanian affairs - a central issue in Panamanian debate over the treaties. How Panama would react to this change remains to be seen.

The Panamanian ambassador in Washington, Gabriel Lewis, was shown the text of the DeConcini reservation by a reporter yesterday. He studied it carefully, but said he could not comment.

The United States has sent Ambler Moss, a State Department official involved in the treaty negotiations, to Panama to explain what has been happening in the Senate.

Because it is a reservation in the form of an amendment to the Senate's resolution of ratification, the DeConcini language should not jeopardize the validity of Panama's earlier ratification of the treaty, according to State Department lawyers. But lawyers in Panama - or politicians, for that matter - could take a different view.

The Senate will consider DeConcini's reservation today, and approval is likely. Its adoption could influence one or more of the undecided votes, and it would assure DeConcini's vote for the treaty.

Yesterday the Senate adopted another reservation that was requested by several other key swing senators. Offered by Sam Nunn (D-Ga.), this one merely states that the treaty's prohibition on the basing of any foreign troops in Panama after 2000 should not preclude the possibility that the United States and Panama might jointly agree to keep American troops there after that year.

Though this has no substantive effect, several senators said it was symbolically important as an indication that the United States did not want to foreclose the continued basing of troops in Panama. The reservation was adopted 82 to 16.

Only the hardcore treaty opponents voted against Nunn, most of them conservative Republicans. The Republican leader, Howard H. Baker Jr. (Tenn.), has risked his political future to fight the reservatives to win support for the treaties. Aides to Baker showed their relief yesterday at the prospect of success in today's vote.

The right wing of the Republican Party has tried to make a test case of the treaty votes, hoping to establish the rule that a Republican with presidential ambitions would have to accept its view and vote no. Baker, who has such ambitions, chose another course, demanding changes in the treaties as the price of his support, and then fighting for them.