James Dobson has made himself a key figure in the forthcoming Senate consideration of the nomination of Harriet Miers for the United States Supreme Court. The prominent evangelical conservative has strongly supported the nomination, saying that undisclosed sources have given him information unknown to others proving that she would be a good jurist.

He acknowledges that Karl Rove, President Bush’s top political adviser, discussed the matter with him. Dr. Dobson needs to share that information with senators who will soon consider Ms. Miers fitness for the court.

The central but sometimes unstated issue is whether Ms. Miers can be counted on to provide a fifth and deciding vote against Roe v. Wade, the 1973 decision that legalized abortion. Dr. Dobson’s statements have raised a question as to whether Mr. Bush somehow broke his promise never to make opposition to Roe a litmus test for his judgeship selections.

Mr. Bush’s repeated “trust me” statements and Dr. Dobson’s unsourced assurances have not been enough to stop many leading conservatives from worrying that Ms. Miers would turn out to be another David Souter. Justice Souter was billed as a reliable conservative when President George H.W. Bush nominated him, but he soon emerged as a key member of the court’s liberal bloc.

Dr. Dobson has thus made himself a likely witness for the Senate Judiciary Committee’s confirmation hearings, expected to start in November. The committee chairman, Sen. Arlen Specter, R-Pa., and several Democratic members want to know exactly what Dr. Dobson has been told about Ms. Miers that made him so certain.

“If Dr. Dobson knows something that he shouldn’t know or something that I ought to know, I’m going to find out,” Sen. Specter told a television interviewer. “If there are back-room assurances and if there are back-room deals and if there is something which bears upon a precondition as to how a nominee is going to vote, I think that’s a matter that ought to be known by the Judiciary Committee and the American people.” He’s right.

How did Dr. Dobson get such influence? He is a psychologist by training and wrote a conservative child-rearing manual in 1970 titled “Dare to Discipline.” In 1972, he launched his radio show “Focus on the Family,” which now claims a weekly audience of 4 million plus many more millions as frequent listeners. The show is carried by 4,000 radio and television stations in more than 40 countries.

He operates from a 77-acre “welcome center” just north of Colorado Springs, Colo., which attracts visitors from all over the world and has a staff of 1,300, including a crew of 120 who answer 10,000 letters a day.

A recent visitor, Christopher Ott, writing in the Internet magazine Salon, credited Dr. Dobson with tapping into the fears of his people for themselves and their families “in a world filled with Darwinists, Sodomites and Disney characters, but also filled with real dangers like violence, cultural and economic changes and spiritual cynicism.”

The Judiciary Committee should call him as a witness to probe the source of his information about Ms. Miers because the public should know what Dr. Dobson knows.