Nuclear power supporters here have all but conceded defeat in a Saturday vote that will be the first referendum on nuclear power in the United States since the Three Mile Island nuclear reactor accident.

"It looks like we're going to get beat pretty good," said Austin attorney John Rogers, campaign manager for the pro nuclear Committee for Economic Energy. "It [the Pennsylvania reactor accident] has hurt us a bunch.

Rogers, who predicted that antinuclear forces will win by "about 60-40 percent," said the nuclear vote was "close to an even race" until the Three Mile Island accident.

He said that accident has been decisive in turning Austin voters against further participation in a four-city joint venture to build two 1,200 megawatt nuclear generators near coastal Bay City, Tex., some 150 miles from Austin.

The anti-nuclear Citizens for Economical Energy also are predicting victory, although by a more cautious to 53 to 47 percent, but maintain they would have won the election even without the Three Mile Island accident.

There are four nuclear energy-related questions on Saturday's ballot here, sparking up an otherwise lack-luster election in which the mayor and incumbent city council members are expected to win reelection.

Proposition 1, which is supported by the city's business leaders and by a 4-to-3 city council majority, would authorize sale of more than $215 million in city bonds backed by the city owned electric system, to finance Austin's 16 percent share in the "South Texas Nuclear Project."

The city has already sold more than $160 million in bonds for the project, which is also funded by San Antonio's municipal power system and by private utility companies in Houston and Corpus Christi.

The additional $215 million in bonds would cover construction cost overruns on the project, now scheduled for completion in 1982, and help purchase a start-up supply of nuclear fuel.

Proposition 2 would authorize the council to sell all of Austin's share in the South Texas Nuclear Project, with the other project participants the most likely customers.

Proposition 3 calls for the sale of $433.9 million in bonds-twice as much as Proposition-1 for building a lignite fueled electrical generating plant, while Proposition 4 would commit proceeds from the sale of Austin's portion of the nuclear project to building the lignite plant. Lignite is a strip-mined low-grade coal common in east Texas.

City officials say Propositions 3 and 4 are on the ballot because the bond compact accompanying the city's $160 million bond sale for the nuclear project in 1973 mandated that those bond funds be spent for power plant construction.

Anti-nuclear advocates, however, contend that the city council's pro-nuclear majority added Propositions 3 and 4 to the ballot as a device to make nuclear power look cheap by comparison.

There is no active campaign support for either Propositions 3 or 4 and most local political observers expect both questions to fail.

As the similar names adopted by the pro- and anti-nuclear forces suggest, the battle over Austin's role in the south Texas Nuclear Project has been fought out primarily along economic lines.

The pro-nuclear Committee for Economic Energy maintains that, construction cost overruns notwithstanding, nuclear power is the cheapest future energy supply available to the city.

More than 70 percent of Austin's power is currently generated from plants fueled by natural gas, they say, and state and federal energy regulations require development of alternative generating sources within the next 10 years.

But the anti-nuclear Citizens for Economical Energy predict those regulations will be lifted in the wake of discoveries of plentiful new supplies of natural gas in the United States and Mexico.

There have been four other votes on nuclear power in Austin since 1972, with each side having won two votes up until now.

If anti-nuclear forces win Saturday, Jones said, local activists will begin efforts to halt construction of the South Texas Nuclear Project altogether, beginning with a demonstration at the construction site in June.