Democrats and Republicans may be worlds apart on most things, but at their headquarters just two blocks away from each other on Capitol Hill, each is confronting the same question: Have political parties lost their purpose?

In the wake of two presidential defeats, the Republican National Committee on Monday unveiled its Growth and Opportunity Project, an effort to give the party engine a top-to-bottom tuneup.

The winning side of last year’s presidential election has been doing some reexamination, too.

This past week has seen President Obama’s campaign operation relaunch itself as Organizing for Action, building a new political machine outside the Democratic National Committee and causing some quiet consternation among party traditionalists.

After Obama’s first election, his campaign organization, then known as Obama for America, decamped to the DNC. But that, as the president acknowledged last week, turned out to be a disappointment, when it proved unable to re-create its magic for the 2010 midterm elections.

“What we don’t want to do is repeat the mistake I think that I believe in 2008 we made, where some of that energy just kind of dissipated and we were only playing an inside game,” Obama told a dinner gathering of about 75 big donors and other supporters of the new endeavor, a comment that rankled some at party headquarters.

Though some Democrats fear that OFA will compete with party organizations for resources, its officials insist that the new operation is designed not to win elections but to advance Obama’s agenda. They add that the president is committed to ensuring the party’s success in 2014, including helping with its fundraising.

Political parties are nearly as old as the republic, performing the basic roles of putting forward candidates for election, explaining their philosophies and then organizing people to vote for them.

But old tools such as patronage jobs do not provide as much influence in a mass-media era when fewer Americans claim a party label. For the past two years, the Gallup organization has reported a record 40 percent of Americans identifying themselves as independent.

“Parties have to continue to redefine themselves to be relevant to the future,” said Jim Messina, Obama’s 2012 campaign manager and head of OFA.

The decline of the parties and their battles to remain relevant are forces that academics and journalists have chronicled for more than half a century. As far back as 1972, the late Washington Post reporter David Broder wrote a book titled “The Party’s Over: The Failure of Politics in America.”

“In the political science field, scholars have had a hard time defining a political party for a very, very long time,” said Daniel J. Galvin, a Northwestern University professor who wrote a 2010 book on the sometimes-fraught relationship between presidents and their party organizations.

But for many of those years, the concern was that the parties were too much alike and philosophically undefined. For instance, if you said “Democrat” in the 1950s, you might be describing a Southern segregationist or a left-wing Northeasterner. Republicans for decades were united primarily by their views on economic issues, and tolerated a broad range of opinion on social matters and on national security.

Now, the opposite is true. Party labels have become a shorthand for a rigid ideological dividing line — Democrats to the left and Republicans to the right.

And the parties’ clout has receded even more quickly in recent years, because of the 2002 McCain-Feingold campaign finance law, which cut off their access to unregulated contributions known as “soft money,” and the Supreme Court’s 2010 decision in Citizens United v. Federal Election Commission,which opened the spigot for money to flow to outside groups.

“The law isn’t the explanation for the weaknesses of the parties, but the law has accelerated their struggles,” said one top Democratic National Committee official, speaking on the condition of anonymity because of the sensitivity of the subject.

Political parties increasingly are outmatched in resources and organization by special interest groups or those, such as tea party groups, devoted more to furthering a cause than to achieving electoral victory.

As a result, the parties are no longer as able to protect their incumbents from ideologically driven primary challenges, to define their messages or even to keep up with technology.

Last year, for instance, the parties spent a total of $228 million on independent efforts to boost their candidates, primarily with ads. That was well less than half the $631 million spent by the “super PACs” that have sprung up since the Citizens United decision, according to figures compiled by the Center for Responsive Politics.

The current campaign finance system “makes it very difficult for the parties to command both the ideological fervor and the money they rely on to deliver a reliable set of services,” former DNC chairman Howard Dean said. He added that he could even foresee a day, somewhere down the line, when “the parties will dissolve.”

The limits on party resources have also produced tension within the parties. Dean, for instance, tried to pour money into a “50-state strategy” to build political operations across the map before the 2006 midterm elections. That sparked a feud with Rep. Rahm Emanuel (Ill.), then chairman of the Democratic Congressional Campaign Committee, who wanted the money to go toward winning seats in swing congressional districts.

There remain, of course, some roles that only a national political party can play. Chief among them is running the process by which a presidential nominee is chosen. But even that has become harder to do.

In the 2012 campaign, for instance, Florida created havoc with the GOP electoral calendar by disregarding the national party’s dictates and moving its primary up into January.

More significantly, the bitter Republican primary dragged on longer than it might have in earlier years. Billionaire benefactors kept the campaigns of former House speaker Newt Gingrich (Ga.) and former senator Rick Santorum (Pa.) on life support well after they otherwise would have folded.

“A long and blistering primary, where people are attacking one another and where the attacks sometimes are not on the mark but are creating an — you know, unfavorable impression — those things are not helpful,” the eventual GOP nominee, former Massachusetts governor Mitt Romney, told Fox News this month.

It has become impossible to rein in those outside forces and the millions they spend, but the RNC is hoping to do a better job of coordinating them. Even that will be a tall order, given the frictions in the party — for instance, between strategist Karl Rove’s American Crossroads super PAC, which represents the establishment, and the insurgents of the tea party, as well as single-issue groups such as the Club for Growth.

At last week’s annual Conservative Political Action Conference, the tension between the Republican Party and movement conservatives was palpable. “The GOP of old has grown stale and moss-covered,” declared Sen. Rand Paul (Ky.), who won CPAC’s presidential straw poll.

With their own party in control of the White House, Democrats face a different challenge. As Messina noted, OFA’s goal is something never attempted before: taking the energy generated by a presidential campaign and transferring it to issues.

Harold Ickes, a longtime Democratic operative, predicted that OFA will have its greatest success in mobilizing Obama supporters to pressure lawmakers on hot-button questions such as immigration and possibly gun control. “Whether they can get that same percentage of people to do something on minimum wage or a fight within the Senate Finance Committee is a different question,” Ickes said.

Over the longer term, the big unknown is what happens to that organization once Obama leaves the political scene. Will its massive infrastructure transfer to benefit the Democratic Party — or simply dissipate?

Georgetown University government professor Hans Noel said that maybe the time has come to redefine the way people think of a party — not as an organization but “as an informal set of actors who try to coordinate to win office or enact policy.”

Still, stronger parties might help unlock the gridlock in Washington, Noel argued, if only to help their candidates get reelected by an increasingly disillusioned public. “They might be more interested in trying to accomplish things,” he said. “They would rein in their extremists.”

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