The Carter administration and congressional leaders threw in the towel yesterday on a key price of electoral legislation, an election-day voter-registration bill.

Conceding that after months of work, the votes weren't there, they decided to get the measure to allow voters to register as late as election day by making it optional for states rather than mandatory starting in 1979, as the bill now requires.

This means the bill would produce [WORD ILLEGIBLE] change, since states have the right now to have election-day voter registration if they choose.

There had already been several "compromises" or concessions made, and there had been heavy lobbying by Vice President Mondale and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.). But the bill was still being opposed by Republicans, some big city organizations. Some Southerners and some Southwesterners.

House Administration Committee Chairman Frank Thompson Jr. (D-N.J.) said the voluntary rather than mandatory provision was being accepted in an effort to "win bipartisan support."

The bill, reported from Thompson's committee over two months ago, is scheduled for House floor action next Thursday.

Although Thompson contended that vote counts showed about four more votes than necessary to pass it in the House, other sources said the votes weren't there. In any case, Thompson said there were not enough votes to shut off a threatened Senate filibuster.

"It's obvious they dont have the votes to pass it," House Minority Leader John J. Rhodes (R-Ariz) said.

"Any state could adopt the method now if they wanted to, so the proposed legislation is meaningless," Rhodes said.

Rhodes conceded that it probably would pick up Republican support, and said he would consider supporting it in this form.

Rhodes had originally endorsed the voter-registration bill, but backed off at the urging of other Republican leaders who felt it would mainly benefit the Democrats and that it could lead to widespread fraud.

While some Republicans were calling it a major defeat for the Carter administration, Rep. Bill Frenzel (R-Minn.), a key Republican on THompson's committee called it "a positive step" and said he would now support it. "They were a little greedy to begin with, and this is a good compromise."

Richard Moe, the aide to Modale who has been shepherding the bill for the administration said, "I don't see how you can portray it as a defeat. If we pass a bill in this form, it's still a significant step forward.

"It's still significant if you establish the concept in law and the encourage the states to opt for the system. We just view it as a longer-term process. We think after states get more experience with it, it will be shown to be a sound and workable system."

Minnesota and Wisconsin have same-day registration. North Dakota has no registration and Oregon and Maine have something close to the system.

Though the bill was reported out of the House committee two months ago, it was shelved for lack of votes. ABout one months ago, Speaker O'Neill announced some "compromise" provisions designed to pick up more votes. They included making it voluntary for 1976, but mandatory in 1979, requiring positive identification to register on election day, and setting up "Satellite" registration and voting places for those who had not registered by election day.

The compromises were designed to answer charges that fraud would be widespread, the concerns of big city organizations that pelling places would be clogged on election day and that some disgusted voters would go home, and concerns of Southwesterners that many illegal aliens might vote under the system.

But Moe conceded that even those provisions did "not entirely" overcome the opposition of groups like the Cook County, Illinois political leaders.

Thompson said he, Moe, Mondale and White House staffers Hamilton Jordan and Frank Moore had met Thursday to discuss the vote situation. He said Carter was told of their decision yesterday and approved it, as did O'Neill and Democrats on Thompson's committee.

Thompson said they were also considering increasing the "incentive" money to be given to the states to register voters under the program. As it is now in the bill states would be given 35 cents per voter if they register them for federal elections only and an extra 10 cents per voter if they register them for both federal and state elections. He said an increase to about 40 cents per voter was being considered, bringing the total cost to about $55 million.

Other pieces in the electoral reform package include extending public fiancing to congressional campaigns, which is likely to pass, removing restrictions on political activity of federal workers, the reforming the Electoral College, both of which are somewhate doubtful.