Several weeks of quietly making the rounds of Senate offices have paid off for civil rights groups and their friends on Capitol Hill. On Dec. 16, the final day before the Christmas break, they unveiled a surprise package: a Senate bill to renew the Voting Rights Act that has more than half the US senators as cosponsors. The bill is exactly the same one that the House passed in October by a wide margin.
Although signing up the majority does not guarantee easy passage in the Senate, it will make renewal difficult to oppose. And it shows that even in its conservative mood, Congress is reluctant to turn its back on the Voting Rights Act, widely acclaimed as the most important civil rights legislation ever passed.
Enacted in 1965, the law protects the rights of minorities to vote and has been credited for registration of hundreds of thousands of black voters in the South. It requires most deep Southern states plus Texas and Alaska to preclear all voting changes through the Department of Justice in Washington to assure that they are nondiscriminatory. And in some areas it requires bilingual ballots.
President Reagan has criticized the House-passed renewal as extreme. He called for easing provisions that allow states to bail out of the ''preclearance'' requirement.
But even without a full presidential endorsement, the renewal may have prevailed.
In all, 58 senators are cosponsoring the bill introduced by Sens. Edward M. Kennedy (D) of Massachusetts and Charles McC. Mathias Jr. (R) of Maryland. In addition to these longtime civil rights supporters, cosponsors include J. Bennett Johnston, the Louisiana Democrat who has led a crusade for a ban against busing for school desegregation. Several other Southern lawmakers are on the list as well as seven Republican committee chairmen.
A pleased Ralph Neas, executive director of the Leadership Conference on Civil Rights, called the bill the ''strongest possible bipartisan measure.'' The only danger to it now would be a filibuster on the Senate floor, he said. More important, the renewal will not have to go into a House-Senate conference where it could be bogged down and weakened.
Senate action on the Voting Rights Act is expected next spring. Major provisions of the current law expire in August.