What does that term even mean today? Polls vary widely on the percentage of voters who can be defined by a centrist position. A recent poll by

Esquire and NBC places this number at 51 percent. A Gallup poll finds that only 34 percent of Americans self-identified as moderates in 2013. Claims from other groups cite this number to be much higher at 60 to 70 percent.

Granted, statistics can be manipulated to support a range of arguments. However, even using the lowest estimates of one-third to one-half of Americans standing in the middle, why has it been so difficult for a third, center-based party to take root?

Third parties and other center-minded movements do exist in America. The core idea behind many of these groups is that a stronger center presence, whether a formal centrist party or a looser coalition of representatives from the Republican and Democratic parties, will change the dynamics of gridlock and dysfunction seen in Congress today. One can only hope it would be this simple.

There is a fine line between groups which promote bipartisan efforts, and those which call for the establishment and growth of a separate third party. The key difference is that bipartisan groups, like Center Forward, No Labels, and the Bipartisan Policy Center, work within Congress to foster better communication and collaboration across party lines.

Center Forward aims to work within the two-party system, and find common ground among Congress and other policy architects to reach common sense solutions. “The center is where we leave our political labels and baggage at the door,” the group states as part of its mission.

No Labels is also about problem solving across party lines. The movement emphasizes that we must say “no” to the status quo which has resulted in ineffectiveness and gridlock.

The Bipartisan Policy Center promotes, as its name suggests, working across the aisle to resolve differences in Washington. The organization stresses balanced and fact-based negotiation, along with advocacy and outreach on a wide spectrum of national policy issues.

These groups are well-established, mainstream efforts, working mostly within government to promote an environment where Democrats and Republicans can work together to solve problems.

At the other extreme is what has become known as the “radical center.”

Ted Halstead and Michael Lind wrote The Radical Center after the 2000 elections, arguing that a monumental shift had occurred among Americans toward moderate positions not represented by the Republican or Democratic parties.

“The American people are well ahead of their political leaders,” Halstead and Lind state.

They argue for a reinvention of American politics.

The idea of radical center politics has its roots as far back as Confucius and Aristotle, but became a more defined concept in the 1960s, when the term “radical middle” was first used in a political context.

Those in the middle can be viewed as ideologically boring -- as fence-sitters. The radical middle, in contrast, stands for action -- for reforms and changes to re-work traditional lines and alliances and achieve progress. However, descriptions like this can easily be confused with a progressive ideology, and some might quickly draw the conclusion that radical middle = progressive = liberal.

Labels can be a barrier to acceptance. Use of the terms “moderate” or “centrist” to describe the middle ground of political ideology is safer territory, even at the risk of sounding non-committal.

Third Way claims to represent Americans in the “vital center – those who believe in pragmatic solutions and principled compromise.” The group proudly says the media has labeled them as “radical centrists” and “incorrigible pragmatists” as they work on an agenda including economic progress, climate change, immigration reform, gay marriage, and gun safety.

Then there are the separate third parties that promote centrist/moderate views by working outside traditional party politics.

The Centrist Party is about “broadening the scope of choice for America.” The party claims that "America is already Centrist,” and expresses the desire to provide a home for those seeking an end to polarized politics.

While its platform is well-defined, the Centrist Party appears to lack candidates. It was difficult to learn from its website whether any candidate for public office has ever run under the Centrist Party banner. Party leaders were unavailable for clarification.

The Reform Party of the United States claims to be the “leading moderate, centrist and populist political party” and that they represent the 60 percent of voters who are ignored by the Republican and Democratic parties in America. Their most notable candidate was Jesse Ventura, who served as governor of Minnesota from 1999 to 2003.

The Reform Party's candidates and officeholders have primarily consisted of state and local offices. The party seeks to shift the balance of power by gaining enough of a share to prevent a single-party majority, and by doing so it will “promote the interests of center-thinking Americans.”

In the end, the battle for the middle will largely depend on whether voters are willing to step outside the comfort zone of the two-party system. Will they take a firm stand to support emerging third parties and independent candidates who represent the center? The ultimate test will be at the ballot box.

This is the fifth in a series of articles about relatively small, lesser-known, grassroots movements hoping to reverse the trend of congressional stalemate, political divisiveness, and the lack of civility in public discourse in America.