(Reuters) - U.S. political independents, who typically occupy the center ground in a closely watched monthly survey of overall consumer attitudes about the economy, have drifted closer this year to the dour views held by Republicans, a potential warning sign for Democrats hoping to hold onto the White House in the Nov. 5 presidential election.

In another indication of the difficulty President Joe Biden faces with voters on the economy - consistently ranked as the U.S. electorate's top concern ahead of the election - the University of Michigan's monthly consumer sentiment survey fell to a six-month low in May. Assessments of the current situation were the lowest in a year and household expectations were the weakest since December.

The breakdown by party not surprisingly shows Democrats - in control of the White House since January 2021 - notably more optimistic than Republicans, a partisan divide that has been routine since the survey began asking for respondents' political affiliations each month beginning at the start of Republican Donald Trump's presidency in 2017. When Trump was in office, it was Republicans who were persistently more optimistic.

Political independents, seen as a critical swing voting block who may well determine the winner in November, have typically hewed closely to the survey's overall score - until this year.

Sentiment among independents more often than not registers as slightly less optimistic about the economy regardless of which party is in power, but since January it has swung materially below the overall survey reading.

On both an absolute and percentage basis, the downside deviation in May - 6.6 index points or 9.6%, respectively - was the largest since the monthly readings have been published. Since January, independents have registered an average reading 5.1 points, or 6.7%, below the overall Consumer Sentiment Index compared with an average downside gap of 1.6 points, or 2.2%, since February 2017, when monthly readings began.

In fact, readings from independents in five of the last six months have been more than one standard deviation below the series average, indicating the gap is both materially outside the norm and persistent. Readings from two of the last three months have been more than two standard deviations below average.

A Reuters/IPSOS poll published on Tuesday showed Biden's overall approval rating this month fell to 36%, the lowest level in about two years.

The economy was picked by 23% of respondents as the most important problem facing the country, making it voters' top concern. Forty percent of respondents said Trump had better policies on the economy versus 30% who picked Biden, while the rest said they didn't know or didn't answer the question.