One of the most important figures in the presidential campaign this fall is a controversial, hypercompetent blonde. She has blazed new paths, is always on message, and has written a best-selling memoir. Friends and foes alike refer to her by her first name only. And she'll play a significant role shoring up the bona fides of her party's candidate among a core constituency that might be wavering.

No, I'm not talking about the Democrats sending out Hillary Clinton on behalf of Barack Obama to reassure white working-class voters. I'm talking about Carly Fiorina, the former chief executive officer of Hewlett-Packard, being sent out to reassure business-class voters on behalf of John McCain. Fiorina has emerged as the most prominent surrogate on economics issues in any of the major campaigns, and her alliance with McCain suggests both his strength and his weakness on the subject.

In July 1999, when Fiorina was named chief executive officer of Hewlett-Packard, she became the first woman to run one of the companies in the Dow Jones Industrial Average. Her rocky, five-and-a-half-year tenure was full of drama: a big, ill-timed merger with Compaq in September 2001, the dot-com/tech meltdown, difficulty managing the board and other executives. When she resigned in February 2005 there was much speculation that the charismatic executive would pursue a career in public life. In March the Republican National Committee named her victory chairman, and she pledged to stump for McCain.

At first Fiorina seemed likely to perform the role that then-Goldman Sachs co-head Robert Rubin played for Bill Clinton in the 1992 campaign. Democrats frequently need business surrogates and market whisperers, rich, successful people who can reassure the CEO class that the candidate won't threaten them much and that he understands the needs of business and the markets. Republicans traditionally haven't required such ambassadors, but McCain does. The Bush fiscal mismanagement of the past eight years and the continuing realignment of the educated and wealthy toward the Democrats have made Republicans suspect on Wall Street. In 2004 an impressive list of executives declared themselves as business leaders for Kerry. And while a list of economists have signed on to McCain's non-reality-based economic program, big-shot CEOs haven't been rushing to endorse him.

McCain is a particularly suspect Republican. He opposed the Bush tax cuts. He has admitted that the economy isn't his strong suit. He's doesn't have an instinctive grasp of how to talk about many of the biggest economic issues.

This is where Fiorina comes in. Here's how NEWSWEEK's Andrew Romano described the McCain-Fiorina dynamic at an event for small-business people in Brooklyn in April: