What makes a leader?
We examine this year's leading presidential candidates through that prism.
Our leadership categories are adapted from standards developed by Fred Greenstein, a Princeton scholar and author of "The Presidential Difference."
Greenstein says the modern presidency is so powerful, voters should take careful stock of the strengths and weaknesses candidates might bring to the job — from their psychology, emotional maturity and vision to the way they process information, manage and communicate.
Using Greenstein's work as a jumping-off point, we assess candidates in the following areas: political skills, communication skills, policy vision, decision-making style and management skills.
The eight profiled candidates have double-digit support in the latest national USA TODAY/Gallup Poll.
Not every presidential candidate would embrace an unpopular war policy or, in the heat of the Republican primary, team up with one of America's best-known liberal Democrats to propose a pathway to legal status for millions of undocumented immigrants.
Or take positions on campaign finance reform, tax cuts and climate change diametrically opposed to his party's activist base.
Or denounce certain leaders of the Religious Right, a powerful influence on Republican politics, as "agents of intolerance."
Or criticize federal subsidies for ethanol, the corn-based fuel additive that is a sacred cow in Iowa, traditional home of the nation's first caucuses.
But John McCain, a four-term senator from Arizona, has risen to national prominence with a reputation as a politician who does it his own way.
"The word 'maverick' was invented for him," says Larry Sabato, director of the Center for Politics at the University of Virginia.
McCain was widely described as an insurgent Republican when he nearly upset GOP front-runner George W. Bush in 2000. He continues to cultivate his independent image, despite his intimate association with President Bush's current Iraq war strategy.
While never wavering in his call for victory and warnings about the consequences of defeat, McCain points out that he alone among the leading 2008 Republican hopefuls has a record of condemning former Defense Secretary Donald Rumsfeld's "terrible mismanagement" of the conflict.
"I'd rather lose a campaign than lose a war," McCain often says when discussing the effect of his Iraq war support on his White House candidacy.
This is the delicate line McCain must walk as he attempts to retain the moderate and independent voters who backed him in 2000 while trying to also appeal to the conservatives central to the GOP nominating process.
McCain's maverick behavior has consequences. He downplays his past attacks on ethanol by joking that he drinks "a glass of ethanol every morning before breakfast," but he continues to lag in Iowa, a state he skipped in 2000.
His attempts to patch up old feuds with the evangelical Christian community turned off many old admirers. At the same time, many social conservatives continue to distrust him.
McCain tries to convince Republican voters that he has learned a lesson from the outcry over his recent collaboration with Sen. Edward Kennedy, D-Mass., on the immigration bill. McCain now promises he will not pursue a temporary-worker program or other perceived benefits for illegal immigrants until the borders are secured.