Within days of the Sept. 11 attacks, the White House upgraded the security clearance of Michael J. Gerson, President Bush's wordsmith, for the dramatic change that lay ahead for his job.

Gerson, a 38-year-old with Armani horn-rims, was hired as Bush's chief speechwriter for his fluency in the strain of Republican education and welfare policy known as compassionate conservatism. Now, he is playing a growing role in preparing the nation for war.

Like Bush, Gerson is learning on the job, helping convert a presidency that was all about tax cuts and faith-based social programs into one that hopes to transform the nation's defense and foreign policies for the first time since the aftermath of World War II.

Gerson is often invited into the Situation Room to soak in the discussion before addresses on terrorism or the Middle East. For Bush's speech to the United Nations last month, Gerson helped establish the just-the-facts tone for the litany of complaints against Iraq's Saddam Hussein.

"We wanted to create an impression, which was justified by the evidence, of inevitability," Gerson said as he worked alfresco at one of the coffeehouses near the White House where he often jots, unrecognized, on legal pads. "The president likes to outline clear and blunt alternatives. This is an organization which is not all that accustomed to that, which added to the drama of the moment."

Indeed, Bush has shelved the "humble foreign policy" he promoted during his campaign and instead plans to use American might to preempt what he considers budding terrorist threats throughout the world. Domestic policy clearly will not be Bush's legacy, or Gerson's. "All these other things remain important," Gerson said. "But you do realize that there's a broader story you're a part of."

Aides to Bush's father recall that he responded to his discomfort with words by distancing himself from his speechwriters, denying them perks and ignoring their advice. White House officials said his son, who saw the disappointing result, learned from his father's mistake and has embraced his writers.

Formal speeches have been so crucial to building Bush's credibility after a gaffe-prone candidacy that scholars are calling Gerson the most influential presidential speechwriter since Theodore C. Sorensen, confidant and muse of President John F. Kennedy.

Anthony R. Dolan, who coined "evil empire" as chief speechwriter to President Ronald Reagan, said Bush's address to Congress nine days after the Sept. 11 attacks and his Sept. 14, 2001, remarks at the National Cathedral ("Our responsibility to history is already clear") will become two of the most memorable presidential speeches in history.

"FDR's 'infamy' line is remembered, but the speech itself is not that distinguished," Dolan said. "Bush's are, and it started before September 11th."

White House officials said Gerson's clout has increased even more now that Bush's longtime adviser Karen P. Hughes is no longer at the White House on a daily basis, even though she remains deeply involved in shaping Bush's image.

"Mike has become the arbiter of what Bush would want," said a person who has sat in hundreds of meetings with him. "When he says, 'It's not going to happen,' there's nobody in the room who's going to say, 'Well, yeah, maybe it might.' "

Two Mondays ago, Gerson was assigned to write an address that would offer vivid evidence to the American public of the risk posed by Hussein, yet try to convince voters that Bush would not attack Iraq rashly. He had to scare people and reassure them at the same time.

As he began the 29-minute speech that Bush delivered Monday in Cincinnati, Gerson wrote that the Iraqi leader was building a fleet of small planes for dispersing chemical and biological weapons. CIA agents reviewing the draft said "small planes" was misleading. Bush wound up warning of "unmanned aerial vehicles," which he later referred to as "UAVs."

Bush remembered from some conversation or briefing that Hussein could make a nuclear weapon with a softball-sized batch of uranium. Gerson checked into that and the government's atomic energy experts finally agreed to "a little larger than a single softball."

Although finessing language to protect intelligence sources and methods is a part of the job that no one had envisioned, Gerson's role has extended well beyond polished rhetoric ever since he joined the Bush campaign a year and a half before the election. Bush formalized that in July, 10 days after Hughes moved out, by elevating Gerson's title to assistant to the president for speechwriting and policy adviser, from deputy assistant to the president and director of presidential speechwriting.

The White House speechwriting office includes eight writers and researchers. Gerson has a West Wing office, an upgrade from his predecessors' quarters in the adjacent Eisenhower Executive Office Building. He attends the 7:30 a.m. senior staff meeting, and often speaks up at the 8:30 a.m. communication meeting, where Bush's daily message is shaped.

Gerson's style is less conversational than his boss's, favoring flourishes and classical devices. "Flowery," the Texans call it. Bush and Hughes have made Gerson adapt to the president's plain-spokenness, which Hughes believes connects with average voters.

The hallmark of Gerson's speeches is the invocation of the vocabulary and literature of faith, and that only increased after Sept. 11, 2001. Gerson, an evangelical Episcopalian who said he is reading a biography of the Apostle Paul for "escape," shares Bush's willingness to talk publicly about the centrality of Christian faith to his life.

The result is a president whose public words are laced with biblical undertones. At Bush's inauguration, he vowed that when Americans "see that wounded traveler on the road to Jericho, we will not pass to the other side." After the terrorist attacks, he told a skittish nation, "God's signs are not always the ones we look for."

Gerson said the White House found that America's broad faith tradition helped foster healing.

"We have tried to employ religious language in a way that unites people," he said. "Martin Luther King did it all the time during the civil rights movement. He was in this long tradition, going back to Old Testament prophets, that says God is active in history and, eventually, he's on the side of justice."

Bush can be harsh with aides who could crowd his spotlight, but people close to Bush say he has only grown more comfortable with the scholarly man whose presidential nickname is "Scribe" or more often "Gerson!"

Gerson had planned a career in teaching and had been accepted at the interdenominational Fuller Theological Seminary in Pasadena, Calif. Instead, he went to work for Charles W. Colson, the Nixon aide turned prison minister, who had seen a column Gerson wrote about Mother Teresa in the college newspaper at Wheaton College in Wheaton, Ill.

Gerson began incubating the notion of compassionate conservatism as policy director for former senator Daniel R. Coats (R-Ind.), now Bush's ambassador to Germany. Gerson caught the presidential-candidate bug and wrote for Jack F. Kemp and Robert J. Dole, then took a two-year detour into journalism when James Fallows, then editor of U.S. News & World Report, recruited him to cover philanthropy.

In 1999, Bush summoned Gerson to his Washington hotel suite during a National Governors Association meeting and hired him on the spot to help develop domestic policy, including an education message that included a federal role.

Gerson rarely watches Bush perform in person -- he caught Monday's speech from his den in Alexandria. But he took Air Force One to West Point, N.Y., in June when the president announced his new military doctrine of preemptive strikes against nations that harbor terrorists or weapons of mass destruction.

Gerson said he knew it would be one of the most important speeches Bush had ever given, and he just wanted to be there. "It only falls to a few presidents in our history to create a new strategic approach for our country that's going to be influential for decades," he said. "We've been forced to think about these things and to articulate them."

"We have tried to employ religious language in a way that unites people," said Michael J. Gerson, Bush speechwriter.