In any organization filled with nothing but ambitious and opinionated people, groups with common interests are sure to come together — and Congress is no different.  

Every member’s Hill career begins by winning election to either the House or Senate, of course, and during the 114^th Congress all of them are caucusing with either the Republicans or the Democrats. But right below those surfaces, the alliances get much more complex, nuanced — and oftentimes contradictory, as lawmakers subdivide into all manner of smaller clusters. Since the nature of the Senate guarantees all members are power centers on their own, the caucuses and other groups to which they belong aren’t all that important. But the ethos is fundamentally different in the House. Since it’s more than four times bigger, and passage of many proposals requires assembling support from many camps, how you’re known and how much leverage you assert has much more to do with which colleagues you hang out with.  

David Hawkings’ Whiteboard: Congressional Factions 

That’s going to be as true as ever in 2016, a year that’s shaping up to be long on campaign navel-gazing and short on important legislating. What does get accomplished will mostly be nibbling at the outer edges of policy, and even that won’t get done without coalitions forming among the regional, ethnic, single-issue and — most importantly — ideological caucuses.  

The power of some of these House alliances are obvious; California has automatic influence because it commands one of every eight seats, and members of the Congressional Black Caucus account for almost a quarter of all Democrats. Several hundred little clubs have formed in hopes of incubating sporadic and narrow victories, from the seven members of the Bourbon Caucus to the 21 in the Caucus to End Bullying.  

Hovering over all of them, however, are the ideological caucuses: a quartet on the Republican side and a trio for the Democrats. Almost 80 percent of members of the House belong to at least one of these groups. And their makeup offers big clues about the nature of the partisan divide in the House and how it will continue to limit legislating this election year.  

For the Democrats, by far the biggest group is the Progressive Caucus, to which three out of every eight members belong. It promotes the traditional liberalism that dominated the party for much of the last century: a robust social safety net, protection of civil rights, higher taxes on the rich as a means for reducing income inequality and a commitment to environmental protection.  

Twenty percent belong to the New Democrat Coalition, which says it’s all about economic growth to benefit the middle class. Formed two decades ago to further the “third way” centrism espoused by President Bill Clinton, it’s the party caucus that makes the most overt outreach to the business community  

And only a relative handful — just 15, or 8 percent — are left in the Blue Dog Coalition, which stands for the most culturally and economically conservative Democratic ideas. Opponents of gun control, abortion rights and unchecked deficit spending tend to find their home there.  

The roster of Blue Dogs was three times bigger less than a decade ago, after a wave of conservative Democrats captured swing districts and helped the party take over the House. Their subsequent slide toward extinction has been countervailed by a 30 percent growth in the Progressives’ membership in a decade — underscoring just how much House Democrats have moved to the left since returning to the minority.  

There’s also minimal overlap between the left and the center. Just three members — Don Beyer Jr. of Virginia, Andre Carson of Indiana and Jared Polis of Colorado — claim membership in both the Progressive and the New Democrat groups.  

On the Republican side, there’s a bit higher percentage of members with feet formally planted in both the mainstream and hard-line camps. But there’s no question which end of the spectrum is dominating.  

The Republican Study Committee was created more than 40 years ago to promote cuts in domestic spending, boosts in defense spending, lower taxes, balanced budgets and the protection of so-called “traditional values.”  

But its membership has almost doubled in the past decade and now stands at 170 — more than two-thirds of the House GOP.  

Much smaller, unsurprisingly, are the two centrist groups: The Main Street Partnership, which has more of a pro-business vibe that calls for putting consensus and accomplishment ahead of ideological purity, and the Tuesday Group, which is socially moderate but fiscally conservative. Four-dozen members belong to both, which isn’t all that surprising given the shared aspects of their mission statements. (The Main Streeters describe themselves as “aligned with the governing wing” of the GOP, for example, while the Tuesday folks pledge their “affirmative obligation to govern.”)  

A bit more surprising is that 1 in 8 Republicans belongs to the mainstream conservative Study Committee, as well as at least one of the more moderate organizations. Eighteen belong to all three, including the No. 4 person in the leadership, Conference Chairwoman Cathy McMorris Rodgers of Washington.  

But not a single member has planted a stake in both the centrist circles and the newest and by far most news-generating faction — the Freedom Caucus, born only one year ago during the 2015 version of the GOP policy and strategy retreat.  

A mixture of libertarian, isolationist and small government conservatives, the caucus members embody the most confrontational aspects of the tea party movement. The group also prides itself on an unusual measure of secrecy. It’s the only one of the seven ideological groups that has no website and doesn’t volunteer its membership list, which now has 39 names.  

Their enthusiastic chorus of “Hell no!” to whatever the mainstream wanted was what forced Speaker John A. Boehner into premature retirement last fall. They are just one-sixth of the House GOP, nowhere close to a majority of the majority that could advance policies proactively. But the roster is more than big enough (actually, they have nine votes to spare) to deny the party leadership the votes necessary for passing legislation with exclusively Republican support.  

That’s leverage that remains more powerful , in the current polarized climate, than strength in the more malleable ideological numbers. That’s the best a caucus can ever hope for.  

Contact Hawkings at DavidHawkings@rollcall.com and follow him on Twitter @davidhawkings. Related: See photos, follies, HOH Hits and Misses and more at Roll Call’s new video site. NEW! Download the Roll Call app for the best coverage of people, politics and personalities of Capitol Hill.