House Democrats and Republicans yesterday rolled out rival budget proposals for a shootout on the House floor tomorrow, with both sides fearing that the House may once again gag on all the choices before it.

"There's at least a 50-50 chance we won't get anything," said House Budget Committee member Leon E. Panetta (D-Calif.), meaning the House could fail for a second time to pass a set of budget targets for the 1983 fiscal year starting Oct. 1. Two weeks ago it rejected eight different proposals, including separate alternatives supported by the House Democratic and Republican leaderships.

In their new budget plan, the Republicans, as expected, proposed to dig more heavily into programs for the poor--plus take a relatively small slice off the top of the president's buildup in defense--in order to get their projected deficit under $100 billion for next year.

The Democrats, by contrast, opted for a higher deficit of $107.5 billion in order to restore funds for a variety of social programs, including $1.9 billion for extended unemployment benefits that were curtailed in Congress' heavy round of budget-cutting last year, as well as smaller amounts for job retraining and high-technology research.

But House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) had hardly announced the Democrats' new plan before some deficit-conscious Democrats were pushing for revisions, prompting late-afternoon skirmishing to fine-tune the spending plan. In the end, the deficit was trimmed by only about $400 million, from $107.8 billion to $107.4 billion.

Even the Republicans, who are conceded by most Democrats to have the best chance of passing a budget during tomorrow's vote, unveiled their proposal without the usual ruffles and flourishes--and with only an implicit nod from President Reagan, who is out of the country.

"There were too many gives, too many takes" for anyone to be "exuberant" about the results, said House Minority Leader Robert H. Michel (R-Ill.) in disclosing the Republicans' proposal, which he nonetheless described as a "truly bipartisan package" that could claim the support of Democratic conservatives as well as all Republicans.

However, one moderate GOP lawmaker said that, while most Republicans would support the party's plan, there would probably be enough defections among moderate GOP "Gypsy Moths" and conservative Democratic "Boll Weevils" to doom it.

As for the Democratic plan, O'Neill said, "I never get in a fight unless I can win it," but then acknowledged that some of his own Democratic colleagues were saying the Democrats cannot pass a budget, even though they have nominal control of the House. "We're winning principles out there," he added, claiming that Democrats in the House and Senate have forced Republicans to retreat on cuts in both Social Security and Medicare.

The new Republican plan, as described by Michel and his conservative Democratic allies, would add back $1.7 billion for Medicare, reducing the GOP-proposed cuts to $3.2 billion for the old-age health care program next year. In contrast, the Democrats would restore $755 million to their earlier proposal for Medicare, adding up to an overall cut of about $1.4 billion.

Both moves were in response to a House vote two weeks ago to restore all funding for Medicare. Both parties also went along with a similar earlier vote to lift proposed limits on cost-of-living increases for federal civilian and military retirees. But only the Democrats acceded to an earlier vote giving federal workers a 5 percent pay increase next fall, while Republicans stuck with their earlier proposal for 4 percent.

The Republicans offset most of their add-backs by cuts in programs for the poor such as food stamps, welfare and Medicaid. The Democrats compensated for their add-backs, which were larger than the Republicans', mostly by adding onto their projected deficit.

In the form finally voted on, the deficit in the Democrats' budget two weeks ago was $105 billion, nearly $2.5 billion less than the newly proposed deficit. The Republicans' deficit had been $103.5 billion, more than $4 billion more than the $99.25 billion deficit that the GOP is now proposing.

Neither party changed its earlier proposal for tax increases: $20 billion for the Republicans and $31 billion for the Democrats. Only the Republicans altered their defense totals, and only by about $350 million.

Republicans claimed their defense proposal would provide a 7 percent "real," or after-inflation, growth in military spending, while the Democrats said theirs would provide 5 percent "real" growth. Reagan had proposed 10.5 percent.

Reagan's own budget, with a deficit projected at $122.2 billion, is being used as the vehicle for the Democratic and Republican alternatives but will be voted on only if the other two proposals fail--and it is not given much chance of passage, even as a last resort.

Office of Management and Budget Director David A. Stockman sat in on the Republicans' budget deliberations over the last few days, and Michel said yesterday he thought "the White House would have to be reasonably satisfied," although he could not be sure until Reagan returns from Europe after tomorrow's vote.

Both sides provide for "reconciliation," under which committees are ordered to cut programs to conform to spending targets. But still at issue yesterday were enforcement provisions for budget targets.

House Appropriations Committee Chairman Jamie L. Whitten (D-Miss.) won House approval two weeks ago for elimination of a critical enforcement tool, under which money bills that exceed targets are not allowed to win final passage, but it had not been determined whether such an amendment would be allowed again this time.