The Senate last night voted 67 to 22 to send a team of expert observers to next month's elections in Rhodesia, a move some senators said they felt would bolster Ian Smith's regime, and other s said they hoped would eventually help undo it.

The resolution to send the observers, heatedly opposed by Sens. Edward M. Kennedy (D-Mass.) and Paul E. Tsongas (D-Mass.), as well as other liberals, was sponsored by a senatorial odd couple: S. I. Hayakawa (R-Calif.) and George McGovern (D-S.D.). Hayakawa supports Smithhs white-dominated Rhodesian government and his proposed "internal settlement" that would lead to black participation in the government. McGovern opposes both.

Pro-Smith sentiment is strong in the Senate, and backers of his government, led by Jesse A. Helms (R-N.C.), want to win a vote to lift long-standing economic sanctions against Rhodesia in the near future.

Hemls, his aides and like-minded senators and staff members were jubilant at last night's news that James Callaghan's labor government had fallen in London. They consider conservative Margaret Thatcher a likley winner in the British elections, which would give them, and Ian Smith, an important new source of sympathy and possible support.

The idea of sending 25 to 50 expert observers to Rhodesia faces serious opposition in the Houses, and may not reach the floor there. Without House action the idea will die.

However, an aide to Helms said last night that several "tax-exempt organizations" are prepared to finance a mission by unidentified observers to Rhodesia if Congress fails to do so. The Helms aide refused to be more specific.

Pro-Rhodesian senators like Helms evidently hope observers would find that Rhodesia's elections were fair and free, considering the civil war environment in which they were conducted, and considering the fact that black nationalist groups boycotted them.

If this were the finding, these senators are prepared to argue that Rhodesia has fulfilled conditions set last summer by Congress for the lifting of sanctions. The United States currently has a ban on trade and other dealings with the Smith regime in conformity with U.N. resolutions.

But McGovern regards the mission of observers as the best way to head off the lifting of sanctions, according to an informed source. The senator is said to beleive that Senate sentiment for lifting sanctions is very strong, and that only a report from experts showing that the Rhodesian election was far from free and unfettered could prevent a Senate vote to lift them.

McGovern is to testify today before the African affairs subcommittee of the House Committee on Foreign Affairs. The subcommittee chairman, Rep. Stephen J. Solarz (D-N.Y.), is cool to the idea.

Helm's aides are trying to pressure the House to act favorably on the observers mission by threatening to seek to lift sanctions in the Senate at once if the House fails to act. Whether this threat will be effective is uncertain. The House would still have to agree to the lifting fo sanctions.

Tghe resolution approved last night by the Senate calls for spending $250,000 for an observer team and security assistance for it.

Critics questioned whether any observer team could operate effectively in Rhodesia, since so much of the country is insecure, airplanes are shot down and guerrillas control large pieces of territory.

Tsongas also argued that the black nationalist guerrillas are winning the civil war in Rhodesia, are determined to interfere with the upcoming elections and are destined to tprevail in the long run whatever the United States does.

Kennedy noted that no other government is sending observers, a situation that could change if the conservatives come to power in time in Britain, and that the U.N. Security Council has asked members to stay away. Kennedy said there was every reason to believe that black Rhodesians would have rejected the constitution on which the elections were based, had they been given a chance to vote on it.

The election date is not certain to be April 20, because the Rhodesian government has reserved the right to adjust the timing in an effort to catch the black nationalists off guard. But if the House does not act very soon there seems little chance thatan observer mission could reach Rhodesia in time.