Prime Minister James Callaghan was weighting a softened offer from the Liberals today that could save his Labor government from defeat on Wednesday.

The 13 Liberal member's of Parliament who hold a strategic position in the struggle for power going on here, have offered to back the government in return for the right to be consulted on all future legislation.

The proposal represents a considerable easing of the terms originally put forward by Liberal leader David Steel on Sunday. He then insisted that Callaghan openly abandon some Labor measures, like the nationalization of banks, and openly promise others, like a revised homerule bill for Scotland.

Now, Steel and the other 12 have dropped this laudry-list approach and are simply asking for the right to give an opinion before any bills are introduced. In effect, the Liberals want the same status enjoyed by leaders of the Trades Union Congress.

Whether Callaghan can accept this, however, is unclear. For some in his party, particularly those on the left, the whole thing will smack of coalition. That has been a dirty word in the Labor lexicon since Ramsey MacDonald abandoned the party in 1931 to form a national government.

In the Commons today, the prime minister was a model of unruffled, avuncular confidence, the pose he habitually strikes. During his regular twice-weekly question period, Callaghan urged his followers to "have no fears" that Tories would soon return to power and added, "I think normal service will be resumed pretty soon."

As for the Wednesday vote, he forecast that an opposition victory is "an unlikely event."

Callaghan was greeted by more than the usual volume of cheers from Labor members of Parliament when he rose. He wise-cracked, "I was hoping it might have been unanimous."

Later, in a television interview, Callaghan said flatly that he was willing to collaborate with another political party - meaning the Liberals - provided that it is "on a basis that preserves self-respect." He carefully declined to say whether the Steel proposal met that test.

Should Callaghan turn down Steel's offer, however, Liberal deputies like Jeremy Thorpe insisted tonight that the 13 would vote solidly to censure the government on Wednesday. If this happens, Callaghan is almost certainly doomed to defeat. The Liberals would join the Conservatives, Nationalists on Scotland and Wales, and most of the Ulster Protestants - more than enough for a majority.

A defeat, in turn, would compel Callaghan to call an early general election in which his party would be a marked underdog.

The Liberals's deputy leader, John Pardoe, also cheered the Labor forces by asserting on television that "one of the few terrible things in the world" would be a government led by Margaret Thatcher. She is the Tory leader and would be the odds on favorite to become Britain's first woman prime minister if the government goes down.

After this, a Labor junior minister said this morning, "I think we'll scrape through on Wednesday."

It there is no agreement with Steel, Callaghan still hopes to peel off a few of the most vulnerable Liberals and a pair of Ulster Protestants. That could be enough to see him through. SOme 628 members of Parliament can vote and Callaghan thus needs 314. He can count on 308 from Labor and two Ulster Catholics. He must find four more from the Liberals and Ulster Protestants.

Some of his ministers are privately telling the prime minister that Labor could even win an election now, a prohecy few detached observers would support. Merlyn Rees, the home secretary, and Eric Varley, the industry minister, are reportedly among the more eruphoric.

They are arguing that Labor's front four - Callaghan, Education Minister Shirley Willliams, Foreign Minister David Owen and Dennis Healey, chancellor of the exchequer - are far more impressive than any team Thatcher can field.

Less euphoric analysts point out that all Thatcher has to do to win is emphasize an inflation rate of more than 15 per cent and unemployment more than 5.5 per cent.

That is why most Laborites are, as one MP said, "praying feverishly" to get by on Wednesday night and put off any election for six months. By then, Britain's economy could be improving.

Today's figures on unemployment for mid-March strengthens this belief. After adjusting for the weather, the rate stood at 5.6 per cent for the second successive month. This is a tenth of a per cent better than the January level and suggests the worst may be over. In addition, the number of job vacancies has increased fractionally for two consecutive months.