But Scott McClellan said the administration does not plan to release any documents from Miers' service as White House counsel that might shed light on her experience with constitutional issues.
"She has deep knowledge of the Constitution and constitutional law," McClellan said at his daily briefing for reporters.
He added that in her job, which Miers has held since early this year, she "has to deal on a daily basis" with matters such as the scope of presidential powers, the Constitution's commerce clause - which gives the federal government a range of authority over the 50 states - and executive privilege.
It is the doctrine of executive privilege that McClellan and other administration officials have cited in saying that documents relating to her White House work are off-limits to the Senate Judiciary Committee, which will conduct her confirmation hearing.
McClellan said yesterday that releasing such data "could have a chilling effect on the ability of the president to receive the kind of open and candid advice that he needs to protect the American people and carry out the other responsibilities that he has."
Before Miers became White House counsel, she was a key aide to the president and before that was Bush's personal attorney.
Bush's nomination of Miers has encountered unexpectedly strong opposition from Republicans as well as Democrats, many of whom contend that her experience does not qualify her for the Supreme Court.
Citing questions about her background, the conservative National Review magazine called yesterday for Miers to withdraw.
"Miers's own career as a lawyer shows a strong tendency to identify with local elites and establishments, to go along with prevailing ideas, and to avoid doing anything that might cause unpleasantness or rock the boat," the magazine's editors said in a posting on its Web site. "These are useful personality traits, but they are not the traits of a [Justice Antonia] Scalia or Thomas - the kind of justice this president led conservatives to expect."
The editorial concluded: "The prudent course is for Miers to withdraw her own nomination in the interests of the president she loyally serves. The president could then start over."
Neither Miers nor the White House has given any sign that she will step aside.
Bush plans an event Monday in the Oval Office with three judges who will vouch for Miers' abilities. In a letter to Senate Judiciary Chairman Arlen Specter, a Pennsylvania Republican, the former Texas Supreme Court justices - Joe R. Greenhill, John L. Hill and Thomas R. Phillips - praised Miers' "legal brilliance."
"Together we represent 34 years of experience on the Texas Supreme Court," the letter said. "We feel confident that we know what it takes to be a justice - Harriet Miers exceeds that mark."
Bush's chief of staff, Andrew H. Card Jr., acknowledged in a TV interview scheduled for broadcast tomorrow on C-Span that White House officials were taken aback by the vehemence of opposition to Miers' nomination from some conservative quarters.
Maura Reynolds writes for the Los Angeles Times.