WASHINGTON — Call it a pox on both the Republican and Democratic houses.

More Americans now call themselves politically independent than at any point in the last 75 years, according to a new poll. The survey also shows that those who do align themselves with a party are more ideological and have become more polarized than at any point in the last 25 years, particularly on issues important in this year’s presidential and congressional campaigns.

Party loyalty, however, only goes so far; neither Republicans nor Democrats say their own party is doing a good job standing up for its traditional positions.

Five months before the November elections, the Pew Research Center poll released Monday sheds light on how the electorate feels about the nation’s two major political parties. And sour seems to be an understatement.

The results indicate a collective thumbs down to both the Democratic and Republican Party, showing that an unprecedented 38 percent of adults rejected both parties and call themselves independents. Only 32 percent now say they are Democrats and 24 percent now call themselves Republicans.

This flight away from the two major political parties began in 2008, a time of intense partisanship as President Barack Obama battled Republican Sen. John McCain for the White House.

Then as now, independent voters are a critical constituency that candidates must win over to prevail in competitive general elections.

Exit polls show these voters have sided with the winning candidate in all but two of the past 10 presidential elections. Independents broke for Obama, 52 percent to 44 percent for McCain four years ago. And recent polling suggests independents are about evenly divided now between Obama and Mitt Romney, his likely Republican rival.

Independent voters also have been on the winning side in congressional contests eight out of nine times since the 1994 election, when Republicans took control of the House for the first time in 40 years.

So both Republicans and Democrats are making serious plays to win them over.

The survey found that the face of the independent voter also is changing, posing challenges for Democrats.

More Hispanic and younger voters — key Democratic voting blocs — say they are politically independent and Republicans are aggressively courting them.

Hispanics who describe themselves as independents have jumped from 31 percent in 2006 to 46 percent now. And nearly half of Americans born since 1981 now say they are independents.

To be sure, 56 percent of Americans still identify themselves as a member of either the Democratic or the Republican parties.

But the parties are pushing out those in the ideological middle.

The vast majority of Republicans, 68 percent, say they are conservative, up from 60 percent in 2000. And the conservative Democrat has become scarce as the share of self-described liberals in the party has grown 10 points since 2000, from 28 percent to 38 percent. As the moderates abandon both parties, the poll finds partisans’ views on the major issues in this year’s campaign have become more deeply polarized since the Pew Center first measured those views in 1987.

The poll measured opinions on 48 different questions about basic political values, and found Democrats and Republicans farther apart than at any point since 1987.

The sharpest differences between partisans fall mostly on the issues at the core of this year’s campaign regarding government’s role and effectiveness: whether regulation helps or hurts business, how involved government should be in people’s lives and whether government programs are effective or wasteful.  Sharp differences also centered on the question of how much of a “social safety net” government should provide — whether government should make sure every citizen’s basic needs are met or take care of those in need even if it means more debt.

Shifting opinions on these issues are not limited to core partisans: Independents who lean toward either Republicans or Democrats are also more sharply polarized from each other than they were 25 years ago, particularly on how much government should do and how effective it is.  

Obama holds a slim edge over Romney in the poll, 49 percent to 45 percent, among registered voters, and the results suggest the sharpest divides between Romney and Obama supporters are over the role and effectiveness of government.

About one-fourth of voters are “swing voters,” or those who are not firmly committed to a candidate. Ideologically, this group is closer to Romney on the social safety net, but closer to Obama on social issues and questions about labor unions. They fall about evenly between the two on the role of government.

The poll also found a liberal shift on social issues in recent decades, with fewer saying they hold old-fashioned values about family and marriage, or the role of women.

The Pew Research Center 2012 Values Survey was conducted by telephone April 4-15 among a random national sample of 3,008 adults. Interviews were conducted by live interviews and respondents were reached on landline and cellular telephones. The margin of sampling error for results based on all interviews is plus or minus 2.1 percentage points.