WASHINGTON - Five months ahead of the US presidential election, US partisan bases have been shrinking and going more polarized, with more Americans identifying themselves as political independents in the past 25 years, according to a new poll released by Pew Research Center on Monday.

As Americans head to the presidential elections this fall, the values gap between Republicans and Democrats is now greater than gender, age, race or class divides, said Pew Research center in the new survey. The average partisan gap has almost doubled since 1987 when Pew started tracking Americans' political values on 48 issues, increasing from 10 percent to 18 percent.

One of the most dramatic shift in partisan values occurred on general assessments of the government's effectiveness and proper scope in recent years. Republicans increasingly feel that regulation does more harm than good since 2007 while Democrats increasingly disagree. The share of Republicans saying the government is too involved in people's daily lives has grown in the same period, while the share decreased for Democrats' part.

The poll finds that the country's partisan polarization has surged in the presidencies of George W. Bush and Barack Obama, with increases recorded nearly across the board.

The Obama presidency has also witnessed the most extreme partisan evaluation of government in the past 25 years when Republicans feel more negatively toward the government and Democrats feel far more positively.

The increasing divergence between Republicans and Democrats coincides with the shrinking of the partisan bases, as the poll show, with a record low of 56 percent of Americans think themselves as either Republicans (24 percent) or Democrats (32 percent).

Previous polls showed 62 percent and 64 percent of Americans identified themselves as a Republican or Democrat in 2008 and 2004 respectively. Democratic affiliation has fallen to 32 percent over the past four years, from a recent peak of 36 percent in 2008 following the 9/11 terrorist attacks. Republican identification fell to 25 percent in 2008 and has not recovered since then.

Thirty-eight percent of Americans identify themselves as independents, the highest in the past 25 years. In fact, looking at data from Gallup going back to 1939, it is safe to say that there are more political independents in 2012 than at any point in the last 75 years, showing a long-term trend that both parties have lost support, said Pew Research.

While Republicans and Democrats are further moving apart over the past 25 years, the polarization also extends to independents, most of whom lean toward a political party.