WASHINGTON (AP) — States that toughened their voter identification laws saw steeper drops in election turnout than those that did not, with disproportionate falloffs among black and younger voters, a nonpartisan congressional study released Wednesday concluded.

As of June, 33 states have enacted laws obligating voters to show a photo ID at the polls, the study said. Republicans who have pushed the legislation say the requirement will reduce fraud, but Democrats insist the laws are a GOP effort to reduce Democratic turnout on Election Day.

The report by the Government Accountability Office, Congress' investigative agency, was released less than a month from elections that will determine which party controls Congress.

The office compared election turnout in Kansas and Tennessee — which tightened voter ID requirements between the 2008 and 2012 elections — to voting in four states that didn't change their identification requirements.

It estimated that reductions in voter turnout were about 2 percent greater in Kansas and from 2 percent to 3 percent steeper in Tennessee than they were in the other states examined. The four other states, which did not make their voter ID laws stricter, were Alabama, Arkansas, Delaware, and Maine.

"GAO's analysis suggests that the turnout decreases in Kansas and Tennessee beyond decreases in the comparison states were attributable to changes in those two states' voter ID requirements," the report said.

The study cautioned that the results from Kansas and Tennessee don't necessarily apply to other states with stricter ID laws. It also found that of 10 other studies that mostly focused on voting before 2008, five found no significant impact from voter ID laws, four found decreases and one found an increase.

The report said that in Kansas and Tennessee, reduced voter turnout was sharper among people aged 18 to 23 than among those from 44 to 53. The drop was also more pronounced among blacks than whites, Hispanics or Asians and was greater among newly registered voters than those registered at least 20 years.

Estimated falloff among black voters was nearly 4 percent greater than it was among whites in Kansas, and almost 2 percent larger among blacks than for whites in Tennessee, the report said.

Young people and blacks generally tend to support Democratic candidates.

A group of Democratic senators including Judiciary Committee Chairman Patrick Leahy, D-Vt., and Vermont independent Sen. Bernard Sanders requested the study and said Wednesday that it confirmed their arguments and reaffirmed the need to pass legislation making it harder to curb voting.

"This study confirms the real impact of Republican efforts to limit access to the ballot box. Playing politics with the right to vote is a shameful practice," said Sen. Chuck Schumer, D-N.Y.

In letters included with the report, the Republican secretaries of state for Kansas and Tennessee challenged its accuracy. Kansas' Kris Kobach said if voting data from 2012 and 2000 were compared, it would show little effect on turnout. Tennessee's Tre Hargett said the GAO report used biased information from a "progressive data firm."

The report also:

—Examined 10 other studies that found that the portion of registered voters with driver's licenses or state ID's ranged from 84 percent to 95 percent. In seven of the studies, blacks had a lower rate of ID ownership than whites;

—Found that in 17 states, the costs of acquiring the required ID's ranged from $14.50 to $58.50;

—Evaluated multiple studies that detected few instances of in-person voter fraud, but said it is difficult to gather data that would produce a reliable overview of the problem.

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