SAN ANTONIO >> In a state where everything is big, the 23rd Congressional District that hugs the border with Mexico is a monster: 8 1/2 hours by car across a stretch of land bigger than any state east of the Mississippi. In 2014, Rep. Pete Gallego logged more than 70,000 miles there in his white Chevy Tahoe, campaigning for re-election to the House — and lost by a bare 2,422 votes.

So in his bid this year to retake the seat, Gallego, a Democrat, has made a crucial adjustment to his strategy. “We’re asking people if they have a driver’s license,” he said. “We’re having those basic conversations about IDs at the front end, right at our first meeting with voters.”

Since their inception a decade ago, voter identification laws have been the focus of fierce political and social debate. Proponents, largely Republican, argue that the regulations are essential tools to combat election fraud, while critics contend that they are mainly intended to suppress turnout of Democratic-leaning constituencies like minorities and students.

As the general election nears — in which new or strengthened voter ID laws will be in place in Texas and 14 other states for the first time in a presidential election — recent academic research indicates that the requirements restrict turnout and disproportionately affect voting by minorities. The laws are also, as in the case of Gallego, reshaping how many campaigns are run — with candidates not only spending time to secure votes, but also time to ensure those votes can be cast.

Thirty-three states now have ID laws, at least 17 of them — including Texas — requiring not just written proof of identity, but requiring or requesting a photograph as well.

Most research suggests that the laws result in few people being turned away at the polls. But a study of the Texas ID requirement by Rice University’s Baker Institute for Public Policy released in August found that many more qualified voters, confused or intimidated by the new rules, have not tried to vote at all.

“What voters hear is that you need to have an ID,” said Mark P. Jones of the Baker Institute, an author of the study. “But they don’t get the second part that says if you have one of these types of IDs, you’re OK.”

A second study, by the University of California, San Diego, concluded in February that the strictest voter ID laws — those that require an identity card with a photograph — disproportionately affect minority voters.

After Gallego’s narrow loss in 2014, researchers from the Baker Institute and the University of Houston’s Hobby Center for Public Policy polled 400 registered voters in the district who sat out the election. All were asked why they did not vote, rating on a scale of one to five from a list of seven explanations — being ill, having transportation problems, being too busy, being out of town, lacking interest, disliking the candidates, and lacking a required photo identification.

Nearly 26 percent said the main reason was that they were too busy. At the other end, 5.8 percent said the main reason was lacking a proper photo ID, with another 7 percent citing it as one reason. Most surprising, however, was what researchers found when they double-checked that response: The vast majority of those who claimed not to have voted because they lacked a proper ID actually possessed one, but did not know it.

Moreover, Jones of the Baker Institute said, “The confused voters said they would have voted overwhelmingly for Gallego.”

While the numbers vary, studies consistently indicate that a modest but significant share of adult Americans — up to 13.6 percent — lack government-issued photo ID cards, like driver’s licenses and university IDs. And the studies consistently show that, compared with whites, the share of minorities without photo IDs is far higher.

In Texas, more than 600,000 people are eligible to vote or are already registered but have no acceptable photo ID, according to an analysis admitted as evidence in 2014 in a federal lawsuit seeking to overturn the state’s law.

Most of the strict photo ID laws have been enacted in the last decade by Republican-dominated legislatures over Democratic objections. Many are in Southern states whose election procedures had been under federal supervision for compliance with the 1965 Voting Rights Act until the Supreme Court invalidated key provisions of the act in 2013.

Gov. Greg Abbott of Texas, who defended the state’s voter ID law against legal challenges when he served as attorney general from 2002 to 2015, says the law’s ID requirements have maintained election integrity and have not disenfranchised voters.

“As the Supreme Court has already held, the voter ID requirement is no more burdensome than other voting requirements, including registering to vote,” Ciara Matthews, a spokeswoman for Abbott, said in a statement. “That minimal burden is far outweighed by the state’s compelling interest in protecting elections and voter confidence in the outcome of those elections.”

Texas officials say they are spending millions of dollars to help voters understand and meet the new ID requirements, including deploying mobile offices to help citizens apply for election IDs. “We just haven’t seen any large-scale problem,” said Alicia Pierce, the communications director for the Texas secretary of state.

But in Wisconsin, Todd Allbaugh resigned as chief of staff to a leading Republican state senator last year after attending a party caucus in which, he said, some legislators “were literally giddy” over the effect of the state’s voter ID law on minorities and college students.

Texas’ photo ID law, which took effect in 2013, remains in place despite being struck down three times in federal courts, and faces another court test later this year.

To enter a voting booth in Texas one of the following is required: a Texas driver’s license; a Texas election or personal ID card; a Texas license to carry a handgun; a military ID; a passport; or a citizenship card. People without the required identification can cast a provisional ballot, but they must produce an approved ID within six days or their ballot will be nullified.

The law’s critics say the list is notable for what government documents it does not include — a Texas university student ID, a tribal identity card or a state employee ID, to cite three examples that could be more likely used by Democratic voters than by Republicans.

Abbott, perhaps the law’s most ardent backer, has said that voter fraud “abounds” in Texas. A review of some 120 fraud charges in Texas between 2000 and 2015, about eight cases a year, turned up instances of buying votes and setting up fake residences to vote. Critics of the law note that no more than three or four infractions would have been prevented by the voter ID law.

Nationally, fraud that could be stopped by IDs is almost nonexistent, said Lorraine C. Minnite, author of the 2010 book, “The Myth of Voter Fraud,” in 2010. To sway an election, she said, it would require persuading perhaps thousands of people to commit felonies by misrepresenting themselves — and do it undetected.

“It’s ludicrous,” she said. “It’s not an effective way to try to corrupt an election.”

As attorney general, Abbott argued that the law’s critics “turn a blind eye to illegal voting and instead rail against voter ID as discriminatory.” That is not so, he wrote in 2012: “States with voter ID laws have seen minority vote participation increase, not decrease.”

That is misleading, said Zoltan L. Hajnal, a political-science professor at the University of California, San Diego, and an author of the study on photo ID laws and minority turnout. The relevant question, he said, is how minority turnout in states with photo ID laws compares with states without such laws.

His study, which relied on a sample of more than 50,000 verified voters collected annually since 2006, reached a stark conclusion. “We’re finding typically that strict voter ID laws double or triple the gap in turnout between whites and nonwhites,” he said.

The Texas law also has a smaller, but potentially significant consequence: Many voters who cast provisional ballots — because they either lack an ID or forgot theirs on Election Day — never return with proper documents to make their votes count.

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