WASHINGTON — Sen. Ted Cruz on Wednesday attacked his Democratic challenger, Rep. Beto O'Rourke, over a tax fraud case involving his rival's mother, marking a deeply personal escalation in what's expected to be Texas' most heated statewide election.

Cruz's campaign questioned O'Rourke's integrity by highlighting the fact that a furniture store owned by the El Paso Democrat's mother, Melissa, in 2010 pleaded guilty to altering financial records to evade IRS reporting requirements.

The Republican sought to tie the tax issue to O'Rourke, noting that the congressman and his mother co-own the shopping center that housed the furniture store.

"Rep. Beto O'Rourke has been campaigning on ethics in government, but he's been hiding his mother's conviction for tax fraud, his personal stake in her furniture company and his profiting from the company," Cruz campaign spokeswoman Emily Miller said in a news release.

O'Rourke, whose mother has joined him on the campaign trail, told the El Paso Times on Wednesday that the store "made a mistake and the issue was resolved." He told the Times that he doesn't want to engage with personal attacks made by Cruz.

"The bottom line is that I'm very, very proud of my mom," he told the Times. "I love her more than I can say and I'm very grateful for everything that she does, including for her entire life running that store, which her mom started in 1951."

Texas Democratic Party spokesman Manny Garcia said Cruz's "cheap shots smell of desperation," noting that Cruz has emerged as a key ally of President Donald Trump even though the mogul launched personal attacks on Cruz's family during the 2016 presidential race.

"What else can we expect from the man that cowered and endorsed Trump after vile attacks on his wife and father?" Garcia said.

Rep. Beto O'Rourke, a Democrat looking to unseat incumbent Republican Sen. Ted Cruz, spoke at a rally May 24 at Mudhen Meat and Greens in Dallas. (Jeffrey McWhorter / Special Contributor)

It's unusual for any politician, much less a front-runner like Cruz, to go after an opponent's mother, particularly so early in a campaign season. The nature of the attack could also portend a nasty shift in a contest that, so far, had followed conventional battle lines.

Cruz has lambasted O'Rourke as an angry, too-liberal-for-Texas Democrat who wants to seize guns and open borders. O'Rourke, meanwhile, has railed against Cruz as an out-of-touch senator driven more by presidential ambitions than a desire to serve Texans.

Personal fireworks had been fairly limited.

Cruz taunted O'Rourke — who is not Hispanic, but speaks fluent Spanish — over his use of his nickname, Beto, as his first name. O'Rourke also agreed with comedian Bill Maher on national television that Cruz is a "giant a--hole," though he later said he regretted that moment.

Great having Mom with us on the trail. Driving the van, stopping for donuts, and joining the important discussions we're having at town halls across Texas. https://t.co/0XtlSkxD0D — Beto O'Rourke (@BetoORourke) March 6, 2018

Enter a Dallas Morning News story published this week that explored the different takes Cruz and O'Rourke have on online sales taxes, a perennial hot-button issue that the U.S. Supreme Court is expected to rule on this month.

The Republican opposes any expansion of states' taxing authority over online retailers, saying it would harm small internet merchants. The Democrat supports that kind of change, saying it would level the playing field in the retail industry and boost state revenues.

Cruz's campaign seized on O'Rourke's reference to a "local furniture store," which the Democrat used as an example of a small brick-and-mortar retailer that is disadvantaged under the current rules since it has to collect sales taxes that its online counterparts may avoid.

"We have to ... make sure we're standing up for those Texas businesses," said O'Rourke, whose general position on the online sales tax issue is shared by the state of Texas.

Cruz's campaign, beyond accusing O'Rourke of pushing for more taxes, used that opening to ding the Democrat over his mother's business record at Charlotte's Furniture in El Paso.

The Republican's team shared documents that detail her company's guilty plea and fine of at least $250,000. They also highlighted an O'Rourke financial disclosure showing that his mother in 2012 — after the tax case — gifted him part ownership of the shopping center that housed the store.

But O'Rourke pointed out that the tax issue has been covered before, including when the El Paso Times late last year chronicled his mother's announcement that the furniture store would close.

The congressman's mother told the Times last year that the closure was not related to her son's campaign, explaining instead that she was "just ready to call it." She also dismissed any connection to the tax case, adding that it "was all cleared up years ago."

"It has nothing to do with what we are here for today," she told the Times.