By David Lightman, McClatchy Washington Bureau (TNS)

WASHINGTON — The roles of the Republican and Democratic parties are undergoing fundamental shifts that threaten their impact on elections and policy.

Built in the 19th century and dominant in the 20th, they now are largely out of date.

They still control the ballot and machinery like the primaries. But they do not hold the loyalty of the people. The largest party in America is no party, with the ranks of people calling themselves independents at the highest level in more than 75 years of polling. The parties do not control the message. People learn about politics from social media instead of traditional means such as mailings or campaign rallies. And the parties are no longer the sole banker of politics. Big-money interests now effectively create shadow parties with extensive networks of donors of their own.

The result: People are tuning out and turning away.

In 2012, average voter turnout for statewide primaries for president, governor and U.S. Senate plunged to its lowest level since the modern primary system became popular in 1972.

It’s a historic change in voter behavior. The Democratic and Republican parties have dominated American politics since the mid-1850s. They grew and prospered as inclusive coalitions that tolerated diverse views for the sake of winning elections, and then consolidating power.

Changes aimed at bringing even more people into the process in the 1970s, such as more primaries and caucuses, have not only outlived their original intent, but they also have wound up allowing unprecedented polarization to strangle party progress. Activists became adept at turning out their own ideological bases, leaving the broad middle on the sidelines.

“Americans’ attachment to the two major political parties in recent years is arguably the weakest Gallup has recorded since the advent of its polls,” the polling organization reported in January.

Just 29 percent called themselves Democrats last year, it found, “making it safe to conclude that the current (number) is also the low point in Gallup polling history.” Republican loyalty was only 1 percentage point above its recent low of 25 percent three years ago.

The bloc of independents reached 40 percent in 2011, and it has stayed at or above that level ever since.

Young Americans are the most indifferent to parties. Nearly half of millennials identified as independents in 2014, Pew found, more than the combined total of those willing to be called either Democrat or Republican.

“I never want to write down that I’m a Republican,” said Rebecca Sorensen, a sophomore at Penn State. She leans Republican but is reluctant to identify with the party because she supports abortion rights.

Historically, children adopted their parents’ political views, including identification with the two major parties. Not anymore.

Millennials get information from sources other than family dinners, neighbors or campaign brochures. If something piques their interest, they turn to Twitter, text messaging, The Skimm and other forms of instant communication.

“If I want to know more, I Google it,” said Jayla Akers, a sophomore at Penn State University.

Political parties are seen as too narrowly focused, too interested in keeping incumbents in office.

“Both parties get so concrete in their values they don’t see any other perspective,” said Bill Corbett, studying to be an auto body technician at Central Pennsylvania Institute of Science and Technology.

For generations, parties welcomed differing views and broader membership.

They now fire up the fringes. Republicans once had a strong bloc of abortion-rights supporters, for example, but in 1976 the party included in its platform support for a constitutional amendment “to restore protection of the right to life for unborn children.”

It’s now unmistakably the anti-abortion party, the comfortable home for conservatives and therefore the party that dominates the South and the Rocky Mountain West. Democrats are the party of the Northeast and the West Coast.

Some in the parties see the growing problem.

The Republican Party’s 2013 self-examination conceded, “Young voters are increasingly rolling their eyes at what the party represents, and many minorities wrongly think that Republicans do not like them.” Since then, party officials have tried hard to bond with younger and minority voters.

Democrats were critical of their own tactics. A party study last year found that too often, many of its candidates “were not connecting with voters and lacked some fundamental infrastructure and support to convey their message.”

“It’s true that today’s multifaceted political landscape changes the footprint of national parties,” said Democratic Chairwoman Debbie Wasserman Schultz.

But, she said, “in the primaries, we set the rules for the nomination and nothing can replace the unique ability of the national parties to effectively organize and mobilize voters,” and their role in the general election is so detailed it “cannot be replicated externally.”

While independents gain clout, so do the big-money groups that now operate as virtual political parties.

Freedom Partners, an organization sponsored by brothers Charles and David Koch of Wichita, Kan., last committed to spend $889 million on politics and policy in 2015 and 2016.

The total would surpass the $404 million spent by the Republican National Committee and the $319 million spent by the Democratic National Committee in the 2012 campaign, according to Opensecrets.org, which monitors political spending.

And that total would rival the $1 billion spent by all three major Democratic Party committees and the $1 billion spent by all three major Republican Party committees.

©2016 McClatchy Washington Bureau. Distributed by Tribune Content Agency, LLC.

Photo: Republican U.S. presidential candidates (L-R) Governor John Kasich, Governor Chris Christie, Senator Marco Rubio, businessman Donald Trump, Senator Ted Cruz, Dr. Ben Carson and former Governor Jeb Bush listen to the U.S. National Anthem before the start of the Fox Business Network Republican presidential candidates debate in North Charleston, South Carolina January 14, 2016. REUTERS/Randall Hill