For immigration bill it's all uphill now as Congress resumes.

The first major immigration reform package in 30 years to pass both houses of Congress hangs by a microscopic thread as the lawmakers begin their trek back to Capitol Hill for a brief session between party conventions.

The Democratic convention here may have dashed hopes for completing work on this most emotional of all the major bills now before Congress. Hispanic Democrats, who spent much of the convention week bitterly fighting the measure, failed to make good their threats of a mass boycott of the first ballot.

But they did succeed in winning a strong commitment from the nominee Walter F. Mondale and his running mate, who only hours before the voting paid a call on the Hispanic caucus.

''We're going to fight it,'' he said of the reform, known as the Simpson-Mazzoli bill. ''We're going to beat it.''

He'll have an ally in vice-presidential nominee Rep. Geraldine A. Ferraro, who will be on hand to lobby her colleagues.

Although Mr. Mondale has long opposed the legislation, now that he has been nominated, his stand puts the House in a dicey situation.

It will be a major test of whether the Democratic House will deliver on the promise made by the Democratic candidate.

''There's no question now, it's got a much tougher row,'' said House Speaker Thomas P. O'Neill Jr. (D) of the bill, which is sponsored by Sen. Alan K. Simpson (R) of Wyoming and Rep. Romano L. Mazzoli (D) of Kentucky.

The Speaker told reporters here last week that the only hope for the reform now is if the Senate agrees to accept the House version, eliminating the need for another vote.

In a telephone interview from Wyoming, Senator Simpson said he would ''consider accepting the House version,'' although he said he had made a promise not to pass the bill until some of the provisions were changed, especially those providing an open-ended guest-worker program, as passed by the House.

''We still have some very interesting options,'' said the senator, who has been working on the reform for six years. But he was clearly disheartened by the protest in San Francisco.

''We knew what was coming, but we just didn't realize the intensity,'' he said, after watching critics of the reform bill denounce it steadily for a week.

Hispanic groups have long charged that the reform, which includes penalties for employers who hire illegal aliens, would bring discrimination against all foreign-looking persons.

''The Hispanic organizations are out of touch with their membership,'' Simpson charged, because the reform would also include legalization for many living here illegally now. The critics ''prattle into the vapors'' without reading the bill and ''pander to the emotions,'' he added.

''All you've got to do is educate the people,'' Simpson said, but he also voiced pessimism about completing legislation this year.

''After you've fanned everything into a raging inferno, just don't tell me you're going to have a thoughtful bill,'' he said.

Under normal procedure, a conference committee of House and Senate members would work out a compromise version, which must then be passed by each house. But it took the House several days of grueling debate, ending in a cliff-hanging vote, to pass the bill the first time, and a second try will almost certainly be more difficult.

With the exception of the immigration issue, the returning Congress will be working chiefly on spending bills to keep the government operating and on passing a 1985 budget resolution.

''If we ever get the appropriation bills out of the way and the budget,'' said Speaker O'Neill, ''we'll close up shop and go home.''

Other bills that could still be considered in the waning days of the 98th Congress include a House proposal to restore some of the Reagan cuts in food stamps, a rewrite of the laws for cleaning up hazardous waste dumps through the Superfund, and natural-gas deregulation.

Congress will meet for three weeks before the Republican National Convention opens Aug. 19.