Mr Bush's little-known White House counsel was raised as a Catholic but converted in mid-life to evangelical Protestantism. She also appears to be a foe of abortion, the most divisive single issue in the culture wars that divide America.
That at any rate is the conclusion being drawn from her membership of the Valley View Christian Church in Dallas, which considers abortion to be murder and homosexuality a sin.
Mr Bush has insisted he had never asked Ms Miers for her views on abortion. But her religious affiliation makes it likely Democrats will be far less inclined to vote for her than for John Roberts, the President's choice for chief justice, who won the backing of 22 of the 44 Senate Democrats and a handsome confirmation last week.
And a tussle between Capitol Hill and the White House is in the offing over the release of documents covering her service there, first as deputy chief of staff and since 2004 as Mr Bush's legal counsel.
Some Republican senators are alarmed that Ms Miers is a virtually unknown quantity, and their demands may be hard to dismiss. "There's precious little to go on," said Sam Brownback, the Kansas Republican and arch-conservative abortion foe on the Senate judiciary committee which will hold Ms Miers's confirmation hearings.
He warned she could be a repeat of David Souter, the present justice who was appointed by the first President Bush in 1990, on the assumption he was a solid conservative. In the event, Justice Souter has invariably sided with moderates on the nine-man bench, to the dismay of conservatives. Ms Miers would replace Sandra Day O'Connor, a centrist among the justices.
"The circumstances seem to be very similar," Mr Brownback said. "Not much track record, people vouching for her, yet indications of a different thought pattern earlier in life."
At Mr Bush's press conference on Tuesday, he tried to dispel these fears. He could vouch that Ms Miers would not shift her views again. "I'm interested in finding somebody who shares my philosophy today, and will have that same philosophy 20 years from now."
But the arithmetic in the Senate could be problematic. Were Democrats to unite against her, Mr Bush will need to hold the 55 Republicans, notably conservatives such as Mr Brownback, firmly in line, especially if Democrats were to provoke a showdown by mounting a filibuster that requires 60 votes to overcome. This could test Republican loyalties when the President's fading popularity could make him a hindrance in the 2006 mid-term elections.
But in a blistering column, George Will, the leading conservative commentator, delivered a broadside against her, urging Republicans not automatically to defer to the President's choice of a woman with few apparent qualifications.
He said that Ms Miers should not be confirmed "unless she suddenly and unexpectedly is found to have interests and talents pertinent to the Court's role". The burden was on her to demonstrate such talents, he added.
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