President Reagan has begun calling wavering senators in support of his selection of William Bradford Reynolds as associate attorney general as the Senate Judiciary Committee heads for a close vote on the controversial nominee today.

Eight of the committee's 10 Republicans are considered certain to support Reynolds, while seven of the eight Democrats are expected to oppose him. For Reynolds to be approved, he must pick up two of the three officially undecided members -- Democrat Howell Heflin (Ala.) and Republicans Arlen Specter (Pa.) and Charles McC. Mathias Jr. (Md.).

However, a 9-to-9 tie would not necessarily kill the nomination. The panel could keep it alive by subsequently voting to send it to the Senate floor without a recommendation.

Reynolds, who now heads the Justice Department's Civil Rights Division, is the prime architect of the administration's civil rights agenda and is opposed by virtually all the nation's civil rights groups. But the undecided senators indicated that their votes would turn on allegations that Reynolds misled the committee in his sworn testimony about a series of issues, including several voting rights cases.

Congressional sources said Reagan has telephoned Specter and Mathias, both of whom questioned Reynolds' credibility at the confirmation hearings. The sources said Specter and Heflin have serious doubts about Reynolds' disputed testimony but still may vote for him on the grounds that the president is entitled to choose the No. 3 official at the Justice Department.

Reagan has sought to dismiss criticism of Reynolds as based on his aggressive stance against school busing and racial job quotas. But Sen. Dennis DeConcini (D-Ariz.), a previously undecided committee member, said yesterday he will oppose Reynolds -- although he agrees with the administration on busing and quotas -- because the nominee engaged in "a consistent pattern of bending and altering the truth."

Reynolds has said he testified truthfully but apologized to the committee for any misunderstandings.

DeConcini said Reagan tried unsuccessfully to call him about the nomination.

Democrats said the vote was too close to call, but Mark Goodin, a spokesman for Judiciary Committee Chairman Strom Thurmond (R-S.C.), was predicting victory.