When the Nixon administration proposed in 1970 that businesses be allowed a faster depreciation write-off on new plant and equipment, there were howls of criticism from virtually every quarter.

Labor denounced the move as an "unconscionable giveaway" to big business. Editorialists clucked that the tax break really wasn't needed. Finally, Congress stepped in to block the change - later enacting it on its own.

This year, the issue of more-generous depreciation write-offs is on the table again, but this time it's the darling - rather than the demon - of the Washington establishment:

Treasury Secretary G. William Miller already has embraced the move, having suggested it himself when he was chairman of the Federal Reserve Board. Miller's successor at the Fed, Paul A. Volcker, also favors the step.

President Carter has cited new investment incentives as likely to be an element in any tax-cut package he proposes in 1980. (Officially, Carter still opposes a tax cut, but has listed what he would like to see in one just in case.)

And both liberals and conservatives in Congress seem to be falling all over themselves to endorse the measure as part of a major tax-cut bill (assuming the two houses push through a big tax cut next year).

"If there's any year we have a chance, 1980 looks like the time to try it," said a business lobbyist who is active in the new push. "I haven't seen anything attract this much support in years. It's almost too good to be true."

The new fever stems largely from the growing public acceptance in the past few years of the motion that the country needs seriously to improve its productivity - and that means investing in new, more modern plants and equipment.

With big-name economists - and many political leaders as well - citing higher productivity as a key way to combat inflation, there's been a shift in popular perception. Business tax cuts no longer are portrayed as bad.

That so much momentum has evolved for an unabashed, flat-out business tqx break is only one unusual aspect of the new push for legislation allowing faster depreciation write-offs.

Another is that, for once, instead of the traditional splintering among a spate of rival measures, proponents appear to be joining forces to throw their support behind a single proposal.

The vehicle is known officially as the Jones-Conable bill, after its original House cosponsors, Reps. James R. Jones (D-Okla.) and Barber B. Conable (R-N.Y.). Both are members of the House Ways and Means Committee.

But the bill has been introduced as well by other key figures in the new push, including Sens. Lloyd Bentsen (D-Tex.) and Gaylord Nelson (D-Wis.). In all, there are 109 cosponsors in the two houses.

And the measure has been endorsed jointly by such diverse business groups as the Business Roundtable, National Association of Manufacturers, U.S. Chamber of Commerce and National Federation of Independent Business.

What the Jones-Conable bill would do essentially is to scrap the present system of linking depreciation write-offs to the useful lives of the assets and replace it with a three-category schedule that would apply economy-wide.

Under the Jones-Conable measure, corporations could write off the cost of new buildings in a flat 10 years, their expenses for eq-ipment in 5 years and outlays for automobiles and light trucks in 3. As a result, the bill has been nicknamed the 10-5-3 plan.

Proponents say the changes would help spur new investment, improving productivity, and also greatly simplify the current depreciation system to make it closer to that of other Western countries.

Under the current depreciation system, businessmen may take a tax deduction each year for a fraction of the cost of new plant and equipment. The timing is related to the estimated life of the item, according to a Treasury table.

The theory is to allow companies to keep more of their earnings in the bank so they can dig up the money to replace the equipment or facility when it finally wears out.

The 1971 changes enacted at the request of the Nixon administration gave businessmen an alternative procedure, called the asset depreciation range, that allows them to write off their capital costs 20 percent faster.

Proponents of a still-faster write-off argue that most other nations permit their corporations to recover their capital on investments more rapidly than the United States. And small firms complaint that the ADR system is too complex for them.

It is still too early to predict flatly that Congress actually will enact the Jones Conable proposal - or any form of legislation that would allow faster depreciation write-offs.

Although proponents point wistfully to last year's cut in capital gains taxes - another once-unthinkable proposal that surprisingly snowballed through Congress last year - skeptics caution that there still are some problems to surmount.

For one thing, the opposition still hasn't been heard from, either on the liberal or conservative side. (No matter what the rhetoric of the day, the really large corporations usually just want a cut in corporate income taxes.)

For another, the cost ultimately may strike a number of legislators as excessive. The capital gains cuts cost $1.8 billion a year in the first year of operation and then swelled to $8 billion or so in later years. But the Jones-Conable bill would be far more costly on both counts - a full $5 billion in the first year of its enactment and $30 billion or more (not counting "reflows" from new activity the cuts may generate) in later years.

On that basis, some analysts still question whether, despite the case for extra investment incentives, speeding up depreciation write-offs really is the most effective way to cut business taxes.

"There's no doubt it would be a good idea to have more investment," said one tax expert involved in the planning this year. "The question is, how good an idea is it? Is the whole effort woth $30 billion a year?"

And some Treasury officials are known to feel that a more modest cutback - involving modifications in the present ADR system - would be less costly and more equitable. It remains to be seen how Miller will come out.

Finally, it's not entirely certain there would be enough room in the budget for the full Jones-Conable proposal. In an overall tax-cut package of, say, $18 billion, it is unlikely that business would get more than $4 billion in all.

If large firms push for a cut in corporate income taxes as well, that would leave relatively little for speeding depreciation any. And ultimately, that's the pattern the bigger companies usually follow.

Nevertheless, there's little doubt at present that the momentum for the depreciation changes is mounting rapidly - to the delight of both business lobbyists and key sponsors.

"Never in my memory has anything this big been taken this seriously," said one long-time observer who recalls the vigorous backlash to the 1971 proposals. Another concluded, "It may be awfully hard to stop."