The Carter administration yesterday formally sent Congress its proposal for a "real wage insurance" tax credit, with an admonition to act quickly on the measure and avoid any temptation to turn it into a general tax cut.

In a letter to the House Ways and Means Committee, Treasury Secretary W. Michael Blumenthal cautioned that either broadening the tax cut or "indexing," as advocated by Republicans, "would be inflationary."

He argued that enacting Carter's "real wage insurance" proposal "promises a direct and major impact on the wage-price spiral" more than "any other single proposal (now) before the Congress."

Blumenthal's warnings were designed to quell the two biggest dangers facing the proposal on Capitol Hill -- that it may be broadened into an across-the-board income-tax cut or used as an inflation adjustment for the tax system.

The Ways and Means Committee is scheduled to begin hearings on the measures Jan. 29, in what all sides concede will be an uphill battle for the administration. Some critics already have discounted the plan as dead.

In an effort to speed action, Treasury officials said yesterday the administration must "know pretty well... by the end of March" whether the proposal has much chance of passage, or the plan won't help much in early bargaining.

Deputy assistant Treasury secretary Emil M. Sunley noted the deadline for the Teamster's union contract, which marks the administration's first critical test with organized labor, is March 31.

He said the administration must have a good idea by then "where this thing is" if it is to be able to hold the tax credit out as an inducement for the drivers to accept a smaller pay settlement.

The administration has been hedging its bets in recent weeks in the face of continually dimming prospects for passage of the measure. Labor Secretary Ray Marshall said Tuesday it wasn't "a linchpin" of Carter's wage program.

The tax credit essentially is designed as a sweetener for labor, offering workers who follow President Carter's new wage guidelines "protection" in case inflation outstrips the 7 percent pay limit they agree to accept.

The measure would allow workers a tax credit of up to one percentage point of their first $20,000 in wages for each percentage point that inflation exceeds the 7 percent wage guideline, to a maximum inflation rate of 10 percent.

If inflation remained at 8 percent, a worker earning $15,000 a year would be entitled to a tax credit of $150 -- minus the tax he would owe on the credit. The credit would be subject to tax at the normal rate each worker pays.

One of the administration's biggest problems has been that organized labor has been cool to the proposal -- a factor that has dampened enthusiasm for the bill in the Ways and Means Committee.

Rep. Al Ullman (D-Ore.), chairman of the Ways and Means panel, has promised to give the measure an airing. But neither Ullman nor other senior Democrats on the committee is pushing it vigorously.

Officials predicted the proposal would provide tax credits to about 47 million workers of the 87 million who file W-2 tax forms -- enticing 21 million who otherwise would have exceeded the guidelines to accept lower pay hikes.

Sunley estimated the cutbacks would trim about half a percentage point from the 8 percent inflation rate that otherwise would occur, leaving inflation for the year about 7.5 percent.

At that rate, he said, the overall wage insurance package would cost the Treasury a relatively modest $2.5 billion -- including $3.3 billion in drainoff from the tax credit and $800 million in taxes collected on the credit.

The tax credit would be granted without question to all workers whose employers certified on their W-2 forms that they were complying with the wage-price guidelines. The standards parallel those in Carter's wage-price program.

Taxpayers would be able to claim the credit by filing in one extra line on their 1040 or 1040-A tax return forms. The tax on the credit would be figured along with that on regular wages. The credit would be added in total wage figures.

The tax credit also would be "refundable." That is, in cases where a worker does not earn enough to benefit from a tax credit, the government will send him the rebate as a direct cash payment.

Part-time workers and farm workers would be eligible for the tax credit, and so would low-wage workers whose pay hikes amounted to less than 7 percent. But self-employed persons would be excluded.

The administration also proposed a small-business exemption under which employers who have 50 or fewer workers would be able to eschew participation in the program if they inform those workers in advance.

Officials said yesterday the plan would not limit the number of rebates for moonlighting workers. Those with two or more jobs -- or working spouses who file joint returns -- may be eligible for two or more credits.

The $2.5 billion net cost was computed on the assumption that inflation will not top 7.5 percent this year. If inflation surged to 10 percent, the tax credit would cost $15 billion.

In a separate action yesterday, the Council on Wage and Price Stability, contending the guidelines program has added significantly to its workload, announced it will seek to add 90 full-time employes in the next few months.

The council also strengthened its procedures for keeping business data confidential by allowing corporations to submit some information on a "conditional' basis until they receive formal assurance that the data will be kept secret.

The council's action was designed to squelch rumors recently that private citiezns could use the Freedom of Information Act to force the government to disclose confidential business data. Officials said this was not true.

The increase in the council's staff would bring the total number of slots in the agency to 233. The council expanded its staff last autumn, to 143 employes from 39 previously, to help organize the guidelines program.