When Congress began looking for loopholes to tighten as part of last year's tax bill, one of the first special breaks to go was the exclusion that allows Americans working abroad to avoid U.S. taxes on the lion's share of their overseas earnings.

At first, it seemed a natural. After all, tax purists argued, these were the persons who already had the glamorous, high-paying jobs - the oil-company engineers and foreign correspondents. Why should they get so generous a tax break as well?

Now, after months of thinking that it was finished with the issue, Congress is facing intense new pressures not only to reverse its 1976 action, but to broaden the tax breaks for U.S. citizens working overseas well beyond what they were before the change.

Moreover, the lobbying campaign has been so effective that all sides concede there will be at least some retreat from the 1976provisions. Congress already has postponed the effective date of last year's cutback until 1977 returns are filed, and may backtrack further later.

The 1976 action pared the amount that Americans working abroad could exclude from their taxable income here to $15,000 - down from the previous $20,000 for most expatriates and $25,000 for those overseas three years or more.

It also required overseas workers to compute their taxes on the remainder as though the initial $15,000 were taxable - in effect, taxing the leftover in a higher bracket. The over-$20,000 earnings previously were taxed at the lowest possible rates.

Finally, the measure forbade workers living abroad from continuing to use the foreign tax credit to offset taxes they paid to other governments on the excluded $15,000 - adding further to the increase in overseas Americans' tax burdens.

Ordinarily, such changes might have semed enough to some disgruntled expatriates. But to top it all off, two recent rulings by the U.S. Tax Court ordered company cost-of-living allowances (for extraordinary expenses such as super-costly housing) made taxable, and the Internal Revenue Service tightened regulations on computing the market value of overseas housing.

This has created a real pinch on some American workers living abroad, sending a number of them packing for home and sending their employers up in arms.

The U.S. & Overseas Employees Tax fairness Committee, a new lobby group set up by the National Constructors Association, contends that the cost of underwriting these increased tax burdens has sent the direct costs of doing business aroad soaring by between 10 and 25 per cent - and has made some U.S. firms unable to compete for lucrative contracts.

Robert M. Gants, vice president of the constructors' group, warns that if American construction firms continue to "lose" overseas business as a result of the new tax provisions, the cost - including a cut in U.S. exports of building materials - could reach $4 billion.

The question is, how deeply is the 1976 change hurting? Although the construction industry clearly has been hit harder than most officials concede the big multinational companies - particularly those that are not as labor-intensive - generally can make up the difference to their workers and still remain competitive. "It's just not an issue there," one tax expert says.

Moreover, many in Congress and tax "reform" groups continue to argue that the previous tax break was too generous. "It's grossly unfair to assume that everyone working abroad ought to be entitled to that large a break," says Robert M. Brandon, director of Ralph Nader's Tax Reofrm Research Group.

The construction industry concedes it was not nearly so vocal when the legislation was being drafted last year. "We simply were asleep at the switch," Gants admits sheepishly. "No one really sat down then and figured out what the impact of these changes would be."

But the industry is making up for it by asking not only for an easing of last year's cutback, but a new set of deductions that som critics complain would exceed even the pre-1976 breaks.

Gants first wants to modify the 1976 law to allow deductions for moving expenses abroad and to bring back the old way of calculating taxes on the taxable portion of foreign earnings. It that doesn't go, industry spokesmen say they will push for liberal deductions for housing, moving expenses, home leave and foreign sales taxes.

The outlook still is uncertain. Although the Treasury has suggested a compromise allowing some deductions for housing and education expenses Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee, reportedly still is reluctant to undo the 1976 tightening. And right now, administration officials say it is unlikely that President Carter will propose any changes in the tax-revision package due out next week.

Still, Ullman, announced yesterday he will begin hearings this morning on whether to postpone the 1976 changes for still another year, and several Ways and Means Committee members hav begun expressing interest in additional changes as well.

Meanwhile, the lobbying goes on, with both Treasury and congressoional sources reporting heavy mail from local construction firms and from U.S. citizens living abroad.