FORTUNE — Perhaps because we feel powerless ourselves, rooting for the underdog, especially in tough times, seems to come naturally. The tendency is even ingrained in popular culture, says Amy Showalter. “In how many movies does the big, powerful — and usually evil — guy win?” she asks. “Not many. That’s partly because the underdog is perceived as more moral, more resourceful, and more deserving of success.”

Still, not all underdogs are created equal. “Just being at an obvious disadvantage won’t guarantee that people will want to see you win,” she notes. Showalter is president of the Showalter Group, a lobbying firm that has organized grassroots political efforts for more than 150 companies, including Southwest Airlines (LUV) and Pfizer (PFE).

For her new book, The Underdog Edge: How Ordinary People Change the Minds of the Powerful…and Live to Tell About It, she analyzed heaps of academic research and dozens of real-life examples to figure out what gives some underdogs what Showalter calls “extreme influence.” Determination, a vivid story of struggling against adversity, and the willingness to try harder than your competition all help. So does standing out from the crowd by stubbornly insisting on doing what the public perceives as right.

You don’t have to be small to get “street cred” as a David against the Goliaths, Showalter says. Consider, for instance, Southwest Airlines. It’s now the largest domestic U.S. carrier, but the company “has always played by the underdog creed, most recently with its ‘Bags Fly Free’ initiative,” Showalter writes. “Southwest lets passengers check two pieces of luggage for free, while others charge $20 or more to check one bag.” Southwest CEO Gary Kelly credits this one underdog move with boosting the airline’s annual revenues by about $100 million.

Showalter gives Ford Motor Co. (F) high marks for underdog thinking, too. “People admire Ford, as big as it is, for taking the scrappier and more difficult path and accepting no government bailout money,” she says. “Especially in a recession, the public roots for any business that refuses to take the easy way and succeeds anyway by sheer grit. It’s inspiring.”

A few of Showalter’s picks for top underdogs of the year:

Rhode Island State Treasurer Gina Raimondo. The rookie Democrat politician “never got the memo about not taking on the state’s powerful public employee unions,” Showalter says. Raimondo led the reform of Rhode Island’s all-but-bankrupt public employee pension system. Her “unbiased approach gave her underdog street cred.”

Denver Broncos quarterback Tim Tebow. “Underdogs must use unconventional tactics to win, and Tebow is the apotheosis of unconventionality,” says Showalter. Tebow, who took the Broncos from a 4-1 record to 7-1, is also “a cohesive team leader.”

Kathryn Stockett, author of The Help. The previously unknown writer “showed grit in pursuing a publisher for her novel despite 60 rejections.” The book came out in 2010, but Stockett makes the list this year because the 2011 movie version grossed over $169 million, making it one of the year’s top hits — and besides, “she writes about underdogs.”

New York gay marriage advocates. Their victory in 2011 illustrates the importance of what Showalter calls “connected converts.” For underdogs to prevail in politics, they need to persuade a few powerful people, who’ve been undecided or opposed in the past, to take up their cause. When Republican Party bigwigs Paul Singer, Cliff Asness, and Dan Loeb got on board and helped sway key state legislators, the movement achieved its goal of legalization.

The Tea Party. After changing the composition of Congress in 2010, “their team leaders keep them focused on results, that is, on electing lawmakers who agree with them,” Showalter notes. “Tea Partiers continue to affect how the powerful in Washington talk, strategize, and vote.”

What about Occupy Wall Street? Showalter says the movement is on her radar as an underdog to watch in 2012, but so far, OWS lacks a coherent action plan and, moreover, needs to win “connected converts” over to its side.

“The research on how to influence people in power all shows that they are impressed by an articulate leader,” she notes. “It might also help win public support if the Occupiers, like the Tea Party people, cleaned up after their rallies.”