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Voter confidence in Obama flattens

President Obama closes the month of May with a -22 score in the NBC News/NBCPolitics.com Voter Confidence Index -- an improvement over last month, but more evidence that voters’ confidence in him has flattened out.

Carolyn Kaster / AP

President Barack Obama walks through Hyde Park in Chicago.

He hit a low of -63 in September following the debt-ceiling crisis. That steadily climbed to -21 six months later by March.

What is the VCI and how is it calculated?

But for the past four months, Obama has been stuck in the 20s: -26 in February, -21 in March, -27 in April, and -22 in May.

President George W. Bush was reelected in 2004 narrowly with a -11 score. Bill Clinton won in 1996 with a +18. George H.W. Bush lost with a -84. Jimmy Carter lost in 1980 with a -72.

Obama isn't in as bad shape as H.W. Bush and Carter, but he isn't quite yet at even Bush's 2004 level.

The VCI is a combination of three questions commonly asked in national polls -- the president’s job approval rating, direction of the country, and generic congressional ballot (which tracks voter preference between parties rather than individual candidates). Equal weight is given to all three questions. We take the difference between two sets of numbers in each question and add them up.

In the VCI, a positive (+) measurement is generally a good sign for the president’s party while a negative number (-) is not.

For more on the VCI, including history, how it's calculated and which polls we use, click here.