The most interesting and (probably) most expensive Senate race in the United States will tell us how much a Republican can shape-shift and win over a liberal electorate.

Just behind Mitt Romney, who defines the genre of the malleable politician, is Sen. Scott Brown of Massachusetts. In 2010, Brown beat a particularly weak Democrat in a special election to replace Ted Kennedy. He is now facing a particularly strong one in Elizabeth Warren, who represented the ordinary Joe as a member of the panel overseeing the bank bailouts after the 2008 financial crisis.

The underlying theme of this senatorial campaign is the candidates' humble beginnings. Warren is a populist from another era, the daughter of a janitor. She worked her way through law school dandling a child on her knee and says she believes that most of what has happened in Washington in the past few years has favored those who broke the economy, not those who were broken by it. It caused her to be a pariah in her party and deprived her of the opportunity to be the first director of the Consumer Financial Protection Bureau, which she created.

Her unpopularity in Washington made her the ideal candidate to take on the bland, play-it-safe Brown. She's a throwback to a time when candidates ran on ideas, not for an office. Before she officially declared her candidacy, Warren gave her theory of life in a few sentences: "There is nobody in this country who got rich on his own," she said. "You built a factory and it turned into something terrific, or a great idea? God bless. Keep a big hunk of it. But part of the underlying social contract is you take a hunk of that and pay forward for the next kid who comes along."

Brown, with his pickup truck and aw-shucks personality, has his own hard-luck story with a pitch to the middle class: His parents divorced when he was just a one-year-old, and he shuttled among relatives after. He was the victim of abuse by a stepfather and a camp counselor. He worked his way through school, at one point posing nude (though strategically) for Cosmopolitan.

Aside from providing Brown with money for law school, the photos provided one of the signature exchanges of the campaign: Warren told a questioner at a Democratic primary debate that she "kept my clothes on" working as a waitress to pay her way through school. "Thank God," Brown said when asked the next day to respond to her comment.

Brown has carefully played his Senate hand to win re- election, voting with his party only 56 percent of the time. Brown voted for the Dodd-Frank financial reform law, which created Warren's CFPB, only afterward working to water it down. He ignores social matters, saying the issues of abortion rights and same-sex marriage are "settled."

That's one reason that last week, with polls within the margin of error, the race returned to the personal. In September, Brown released a campaign ad attacking Warren for listing herself as American Indian ("Fauxcahontas" became a Twitter hashtag) in a Harvard faculty directory. To Warren's supporters, she was simply repeating family lore, the usual embroidering of ancestral roots. To Brown's supporters, she was trying to get special treatment as a minority and win the duel over who had the harder upbringing.

Then, earlier this month, Brown made his own questionable claim on a far more tender subject. At a debate Brown said he "served in Afghanistan." True enough: In the summer of 2011 the senator took his two weeks of National Guard training in Afghanistan. To thousands of young men and women in the active military, however, serving in Afghanistan means risking life and limb, not meeting with generals, ambassadors and other officials. Brown's calculations even extended so far as capturing his emotional return from war, complete with wife embracing her long-lost hero dressed in camouflage, for a TV ad.

His act of cloaking himself in the sacrifice and glory of war is all the more curious given that he introduced the Stolen Valor Act to penalize the self-aggrandizing "faux-fighters" who would put themselves in the company of those who had truly served their country.

Just as Brown wasn't at the center of the war, he wasn't at the center of the passage of Dodd-Frank. But as he would have it, "It never would have passed if I wasn't the deciding vote on financial reform."

Nor would it have been watered down. Brown's compelling personal story aside, it may take more than posing as a soldier and driving a pickup to be seen as the Protector of the Little Guy - especially when you're up against the Woman Who Stood Up to Wall Street.