▶ Watch Video: Texas primary election sets stage for November midterms

Thousands of Texans who tried to vote by mail during this month’s primary elections had their ballots tossed out, many of which were rejected because of issues that voters had in trying to comply with the state’s new voting law. 

A CBS News analysis of election data has found that at least 16,800 mail ballots were rejected across 14 of the 25 counties with the most registered voters. That’s about 15% of all mail ballots returned in those counties. 

The rejections came in the first major election since Texas Republicans passed a sweeping new election law, known as SB1, after months of objections from Democrats and voting rights advocates. Republican Governor Greg Abbott signed the bill last September. 

County election officials told CBS News that the provision in the law that gave voters the most trouble was a requirement that they provide a driver’s license number, personal ID number or the last four digits of their Social Security number when applying for and returning a mail ballot. The number that voters provide must match the registration that’s on file. 

County election officials have told the Texas secretary of state’s office that the “vast majority” of mail-in ballot rejections stemmed from the failure to provide proper identification information, according to Sam Taylor, assistant secretary of state for communications at the Texas secretary of state’s office.

Harris County, the country’s third most populous county and home to Houston, had 6,888 mail-in ballots rejected of the 36,878 returned due to issues complying with the new voting law,  according to the Harris County elections department. That’s nearly 19% of all returned mail ballots. Just 31 ballots were rejected for reasons not related to the new voting law.

The number of mail ballots rejected this year far exceeded the rejection rate from the last midterm election primaries. In the 2018 primaries, officials said that just 135 mail ballots were rejected of the 48,473 mail ballots cast, a rejection rate of 0.3%. 

Harris County’s elections department says it doubled the staff dedicated to voter outreach and contacting voters if there were issues. But ultimately, only 849 people corrected their ballots — out of the 7,750 flagged for rejection. 

Harris County Elections Administrator Isabel Longoria announced earlier this week that she would resign from her position on July 1 after county leaders criticized her handling of the March primary. 

But the problems weren’t limited to Harris County. In Bexar County, which is home to San Antonio, nearly 22% of the 18,000 mail ballots returned were rejected. 

In the Dallas-Fort Worth metroplex, Collin County rejected about 14% of all mail-in ballots returned. Bruce Sherbert, the Collin County elections administrator, told CBS News that “virtually all rejections were due to the new ID requirements under SB1.”  

Also in suburban Dallas-Fort Worth, Denton County rejected nearly 17% of all mail ballots. Of those that were rejected, 84% were due to ID issues, according to the county elections department. 

In Austin’s Travis County, a majority of the 896 mail ballots that were rejected — about 8% of the total cast — had issues complying with the new ID requirements, according to a Travis County clerk’s office spokesperson. In Williamson County, just north of Austin, 73% of the 521 rejected mail ballots couldn’t be counted because of ID issues. The next most common reason for rejection was returning a ballot past the deadline. 

Not all of the largest counties saw such high rejection rates. Brazoria County, near Houston, rejected 3.6% of returned mail ballots. 

Texas already has strict requirements governing which voters can cast mail ballots. In order to be eligible to cast a mail ballot, a voter must be at least 65 years or older, sick or disabled, expecting to give birth within three weeks of Election Day, out of the county during early voting and on Election Day or confined in jail but otherwise eligible to vote. 

There were signs of trouble for election officials in the weeks leading up to the primary when high numbers of mail ballot applications were rejected for failure to comply with SB1. Once ballots went out to voters, some counties flagged more than 30% of returned ballots for rejection, but were able to bring those numbers down through aggressive efforts contacting voters to fix their ballots. 

In Webb County, located along the Texas-Mexico border, officials ultimately rejected 31 of 734 returned mail ballots, about 4%. Webb County Elections Administrator Jose Luis Castillo told CBS News that 112 ballots were initially flagged for rejection, mostly because the ballots were incomplete; not all of the new requirements were initially met.

“(Voters) struggled, but it’s more of an educational thing,” Castillo told CBS News. “They’re used to the old system, not the new requirements or the new law.”

The Texas secretary of state’s office encouraged voters to update their registration information online to ensure that they have all forms of identification associated with their registration. State and local officials also encouraged voters to fill out both ID numbers before returning a ballot. 

Taylor, who is with the Texas secretary of state’s office, said state and local officials will focus a large amount of voter education on the new requirements for mail voters moving forward. 

“While in years past we have focused our voter education efforts on in-person ID requirements, this year we are also devoting a significant portion of our voter education campaign to enhancing awareness of the new mail-in ballot ID requirements,” Taylor said in a statement. “We are confident we have the data and research we need to apply any lessons learned during the primary to an even more robust voter education campaign heading into the November General Election.”