President Carter's proposed $50-per person tax rebate barely survived two test votes in a skeptical Senate Finance Committee yesterday, and several Democrats said they finally supported it only out of elementary party loyalty.

Had they voted the way they talked, the rebate would have lost.

The two tests came on motions by the committee's Republicans to replace the one-time-only rebate with "permanent" cuts in tax rates, an alternative they said would do a far better job of reviving the economy. Both motions were defeated by partly-line votes of 10 to 8, with all 10 Democrats on one side and Harry F. Bryd Jr. (Ind. Va.) and the seven Republicans on the other.

The committee also approved, by 9 to 7, a proposal by Sen. Bob Packwood (R-Ore.) that would cut the taxes of most taxpayers classified as heads of households - unmarried taxpayers with dependents.

Heads of households who do not itemize their deductions are currently allowed the same standard deductions as single taxpayers. Effective this year, the Packwood proposal would let them take the higher deductions given married couples filing joint returns.

That would save an estimated 3.4 million taxpayers an average of about $107 each - a total revenue loss of $363 million a year. The House version of the tax bill, however, contains no similar provision, and Packwood's amendment could die either on the Senate floor or in conference.

Most taxpayers classified as heads of household are divorced or otherwise husbandless women with children, and Packwood argued such women need at least as much tax help as married couples who may or may not have children. Congress is presently changing the standard deduction, but under present law couples filing joint returns can deduct up to $400 more than single taxpayers, and under the tax bill the House passed last week that difference would be $600.

The committee is scheduled to continued its work on the tax bill today.

Carter proposed the $50-per-person rebate in January in addition to more orthodox tax cuts for individuals and business. He said the rebate was the best way to give the economy a needed quick fix without permanently giving up revenues the government might need in future years.

But Republicans and other critics said one-time rebates of only $50 per person - $200 to a taxpayer with three dependents - were too small to do much good. The rebate, as Sen. John C. Danforth (R-Mo) observed yesterday, has become "the subject of jokes."

In the House, a Republican floor amendment to substitute rate cuts for the rebate narrowly lost, 219 to 194.

On the Senate floor last week Chairman Russell B. Long (D-La.) of the Finance Committee also took a swing at the rebate idea. The Senate was debating Carter's proposed cut-off of funds for 19 water projects and Long said those threatened projects would do the economy far more good than the rebate.

Before the committee voted yesterday, however. Carter met with Long and spoke by telephone with several other wavering Democrats, asking their support. Long also had Chairman Charles L. Schultze of the Council of Economic Advisers come up and make the case for the rebate one final time before the vote yesterday morning. Even so, four Democrats - Abraham A. Ribicoff (Conn.), Floyd K. Haskell (Colo.), Daniel P. Moyniham (N.Y.) and Lloyd M. Bentsen Jr. (Tex.) - said afterward they had doubts.

"I support the administration on this rebate with a great deal of skepitcism." said Ribicoff. "In all the years I've been on the Finance Committee I've never known any group of economic advisers to any President to be correct." But he said the new administration deserved the temporary benefit of the doubt, and he voted aye.

Meanwhile, the Senate Appropriations Committee balked yesterday at appropriating $3.2 billion for $50-per-person payments Carter has proposed and the House has approved to Social Security recipients and other groups not covered by the rebate. But if the full Senate approves such payments in the tax bill, as expected, the $3.2 billion will be restored.