CIVIL RIGHTS advocates got an early Christmas present with the announcement that 61 senators have cosponsored legislation to extend the Voting Rights Act. The bill they have joined to support is identical to the one passed by the House by a vote of 389-24 in October. Key provisions of this landmark law were due to expire next year, and there has always been a clear majority in the Senate to support extension. The significance of last week's announcement is the magic number 61, exactly one more than is needed to stop a filibuster. Even better, the supporters of the bill are a bipartisan group composed of 21 Republicans and 40 Democrats. A number of prominent southern legislators, including Sens. Bumpers, Sasser, Hollings, Bentsen and Long, are on board.

When the Voting Rights Act was first passed in 1965, it was hailed as the most important piece of civil rights legislation of the century. Because it was preceded the year before by the comprehensive law that desegregated public accommodations, schools, jobs and all programs receiving federal assistance, that was quite a claim. Proponents believed, however, that any injustice would be corrected so long as the franchise was protected. If minorities were free to vote and to use their political muscle, they could effect change peacefully and command the respect of elected officials. The 1964 act, as someone said, distributed a lot of fish, but the 1965 act let its beneficiary do the fishing. Has it worked?

Two measurements are relevant: minority voter registration and minority office-holding. The figures are impressive. Before 1965 only 29 percent of black residents of the covered states were registered to vote. Most of these states now have more than 50 percent of their minorities registered. In these same states, the number of elected black officials has increased from 158 to 1,813 in the last 12 years. This is significant progress, but the numbers still fall short of those measuring political participation by whites in the same areas. This may be due to any number of factors having nothing to do with race. Thus, the law allows jurisdictions that can show absence of discrimination and a real effort to improve minority participation in the political process for a 10-year period to obtain an exemption from the act. If the low figures are due to residual prejudice, however, the law is still needed and could be used for another decade.

In response to a question at Thursday's press conference, President Reagan stood by an earlier administration position that the law should require proof of intent to discriminate by local jurisdictions and not just the actual fact that discrimination has occurred. He expressed the fear that an "effect" test rather than an "intent" test would lead to mandatory proportional representation in elections. The drafters of the House bill went to some trouble to avoid this misapprehension, however. They added the following language to Section 2 of the bill: "The fact that members of a minority group have not been elected in numbers equal to the group's proportion of the population shall not, in and of itself, constitute a violation of this section."

The president has impressed many of his adversaries by quickly recognizing mistakes and cutting his losses. The school lunch and early Social Security proposals are examples. The fact that 61 senators have cosponsored the House bill should assure him that support for the bill is broad and deep. Now is the time to stop objecting and join the celebrating.