DES MOINES, Iowa -- They are barely blips in presidential polls and their campaign cash is scarce. Some are running on empty, fueled mainly by the exposure that comes with the blizzard of televised debates in this election cycle and interviews they eagerly grant to skeptical reporters.

Yet the second-tier contenders for the Republican presidential nomination soldier on. They argue the race is far from over and that anything can happen with polls showing a wide-open race in Iowa five weeks before the Jan. 3 caucuses.

Former Pennsylvania Sen. Rick Santorum is typical when he resists conventional wisdom that only those with a lot of cash and a big campaign can win.

"I feel like I'm doing what I'm supposed to be doing, and I feel like I'm making a difference in the race," said Santorum, who barely registers in state surveys despite having campaigned in Iowa for more than a year.

More than energy and determination, also-ran hopefuls rely on particular issues, free media and prospects for the future to drive them to keep going. With polls and money putting names like Mitt Romney, Newt Gingrich and Herman Cain atop the field of Republican rivals, there's a crop of others likely to remain in the race until voters have their say.

One force in that is the fluidity of this year's contest.

Minnesota Rep. Michele Bachmann was among the many contenders who surged when they got into the race but then plummeted in the polls.

"I guarantee you, with everything within my being, I have the backbone," Bachmann said. "I'll put my backbone up against any other candidate in the race."

That includes Texas Gov. Rick Perry, who is adamant that he's not giving up, even as his campaign flails and his once-flush bank account suffers following a series of debate missteps that has some of his fund-raisers questioning his viability. He -- like Bachmann, Santorum and former Utah Gov. Jon Huntsman -- are barely blips in many surveys.

The nomination is not the only prize. Texas Rep. Ron Paul's libertarian views energize a small but loyal base. Santorum uses his platform to hammer his anti-abortion stance. Bachmann just released a book whose sales could see a boost ahead of the Iowa caucuses.

It's not unusual for lesser hopefuls to stay long after they have fallen out of favor. The structure of the race in Iowa and other early voting states like New Hampshire and South Carolina is designed to make it possible for them to keep going because the states are relatively cheap places to campaign, and they value hand-to-hand campaigning over pricy TV ads.

"In Iowa, you can sleep on people's couches and hang on for a long time with very little money," GOP strategist Rich Galen said. "You can live off the land in Iowa. You can't do that in Florida."