By TRIP GABRIEL

NILES, Ohio — Just weeks before elections that will decide control of the Senate and crucial governors’ races, a cascade of court rulings about voting rules, issued by judges with an increasingly partisan edge, are sowing confusion and changing voting procedures with the potential to affect outcomes in some states.

Last week, a day before voting was scheduled to begin in Ohio, the United States Supreme Court split 5 to 4 to uphold a cut in early voting in the state by one week; the five Republican appointees voted in favor and the four Democratic appointees against. Cases from North Carolina and Wisconsin are also before the court, with decisions expected shortly, while others are proceeding in Texas and Arkansas.

The legal fights are over laws that Republican-led state governments passed in recent years to more tightly regulate voting, in the name of preventing fraud. Critics argue that the restrictions are really efforts to discourage African-Americans, students and low-income voters, who tend to favor Democrats.

Ohio reduced early voting by a week early this year, including one day of Sunday voting when black churches conduct “souls to the polls” voter drives. An appeals court composed of three Democratic appointees had ruled that the cutbacks violate the Constitution’s equal protection clause and the Voting Rights Act, and proponents went to the Supreme Court. The result, here as elsewhere, has been an increasingly charged environment in which voting rules have been as enmeshed in partisan politics as the races themselves.

“The new voter suppression in the 21st century is all this voter confusion,” said Nina Turner, the Democratic candidate for Ohio secretary of state, an office that administers elections.

Her opponent, the Republican incumbent Jon Husted, said that Ohio still had 28 days of early voting, more than the majority of states, and that only “a few extreme voices on the political left” opposed the one-week cutback.

In Wisconsin, voting officials are scrambling to enforce a voter identification requirement after the United States Court of Appeals for the Seventh Circuit declined to take up the case last month. It could affect the tight race between Gov. Scott Walker, a Republican who supports the voter ID law, and Mary Burke, his Democratic challenger. Civil liberties groups appealed the case to the Supreme Court, predicting “chaos at the polls” because there is not enough time for 300,000 registered Wisconsin voters without acceptable IDs, many of them minorities, to obtain the documents. A Marquette Law School poll late last month found that 18 percent of likely voters did not know an ID is needed to vote.

In North Carolina, where a crucial Senate race is underway, Gov. Pat McCrory, a Republican, is appealing to the Supreme Court a lower-court ruling that blocked parts of the state’s voting law, one of the country’s most restrictive.

Courts in Texas and Arkansas are also expected to rule shortly on voter ID laws, which were pushed by Republicans in the name of ensuring the integrity of elections, and opposed by Democrats who argue they suppress turnout. Numerous studies have found that in-person fraud, which ID laws are meant to prevent, is almost nonexistent.

There is less evidence of whether the laws suppress turnout, but one survey of research estimated that participation by registered voters declined by 2 percent, theoretically enough to sway very close races.

Richard L. Hasen, an election law expert at the University of California, Irvine, said that an ideological split in judges’ rulings, which once seemed less of a factor, “has re-emerged in 2014.”

“I don’t believe that any of these judges are deciding cases because they want to help, in quotes, their party win the election,” Mr. Hasen said. “Rather I think people who are more conservative based especially on what they hear in conservative media are more disposed to believe voter fraud is a major problem and the requirements are no big deal. Where liberal judges hear the laws are simply an attempt to stop poor minority voters from voting.”

Mr. Husted, the Ohio secretary of state, defended the reduction in early voting, which eliminated the so-called Golden Week when residents could register and vote at the same time. The bipartisan Ohio Association of Election Officials recommended the change. Mr. Husted said lawmakers closed the potential for “fraud and abuse,” including by out-of-state campaign workers who flood Ohio every four years. He acknowledged, however, that a study he conducted showed fraud was “very rare.”

“It’s not like we eliminated early voting,” Mr. Husted said, noting he had mailed forms to every registered voter to request an absentee ballot. “We still have a more expansive menu of opportunities than almost any state in the country.”

Ms. Turner, his Democratic opponent, cited a study that in Cuyahoga County, which includes Cleveland, African-Americans account for 28 percent of all votes in the 2008 presidential election, but 78 percent of early in-person votes. Absentee mail-in ballots were used mostly by whites.

“What is it about Golden Week and extra hours on weekends and evenings that Republicans hate so much?” Ms. Turner said. “These laws really are about voter suppression and they know it.”

In North Carolina, Senator Kay Hagan, a Democrat, is in a close race for re-election with Thom Tillis, the leader of the State House, which passed its voting restrictions last year. In blocking parts of that law last week, the United States Court of Appeals for the Fourth Circuit ordered that residents must be able to register and vote on the same day, and that ballots accidentally cast in the wrong precinct must be counted. Judge James A. Wynn Jr., an Obama appointee, noted in his ruling that African-Americans in the state used early voting at twice the rate of whites, and they moved more often, making it more likely that they might vote in the wrong precinct. “Why doesn’t North Carolina want people to vote?” the judge asked.

In its emergency appeal to the Supreme Court to reinstate the law, the state’s Republican administration made the same argument that Democrats have in other states — that unacceptable confusion would result from the “unprecedented last-minute change” in voting rules.

Paradoxically, it was Republican-leaning voters who used to cast most early votes in presidential battlegrounds like Florida and North Carolina. But starting in 2008, Obama field workers encouraged supporters to register and vote early. The rate of African-American early voting exceeded that of whites for the first time. Barack Obama became the first Democratic presidential candidate to carry North Carolina in over three decades.

After years of looser voter laws, there has been a “major shift” since the 2010 election: 18 Republican-controlled states acted to restrict voting, according to the Brennan Center for Justice at New York University.

“We don’t have a lot of experience with people cutting back on early voting, but we do in Florida in 2012 and the results were disastrous: There were long lines at polls across the state,” said Wendy R. Weiser, the director of the Democracy Program at the Brennan Center.

In Wisconsin, as in Ohio, Democrats are trying to turn a setback in the courts into a rallying cry to motivate their base. The appeals court decision last month on Wisconsin’s voter ID law, which stayed an injunction that kept the law from taking effect, came from five justices appointed by Republican presidents. On Monday, three of those judges took another step, declaring the law, enacted in 2011, to be constitutional.

Mike Tate, the chairman of the Democratic Party of Wisconsin, said the purpose of the law was to keep constituencies who largely vote Democratic — students, African Americans and those with low incomes — away from the polls. But he said the law and the court rulings had the potential to have the opposite effect, riling up groups who usually sit out midterm elections like the one in which Governor Walker faces a fierce battle for re-election.

“This could have a net effect of putting more votes on the board for Mary Burke,” he said.

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