Democratic vice presidential nominee Sen. Tim Kaine (D-Va.) slammed Donald Trump during the vice presidential debate Tuesday for not paying federal income taxes.

Seizing on the New York Times’ bombshell story that the Republican presidential nominee took a $916 million personal deduction in 1995, Kaine said that by not voluntarily releasing his tax returns “he has broken his first promise.” Trump initially said he would release his tax returns, before backtracking and citing a series of shifting explanations for why he would not release his returns. Democratic nominee Hillary Clinton has released her tax returns, as have Kaine and Trump’s vice presidential running mate Mike Pence; it is the standard practice for modern presidential candidates.

The Times story was based on three previously unreported pages from Trump’s 1995 tax returns that were anonymously mailed to reporter Susanne Craig in September. Tax experts who analyzed the documents said they could have allowed Trump to pay no federal income taxes for 18 years.

Even before his partial tax return was published, the Republican nominee all but invited that line of attack. At last week’s debate, Clinton suggested Trump hadn’t released his tax returns because they would show how little he paid. “That makes me smart,” Trump quickly retorted.

Chip Somodevilla via Getty Images Democratic vice presidential nominee Sen. Tim Kaine (D-Va.) criticized Donald Trump for avoiding paying federal income taxes for many years.

At the vice presidential debate Tuesday, Kaine asked whether Trump meant it’s smart not to pay for the military and teachers. “I guess that makes the rest of us stupid,” Kaine said.

After initially dodging the question of why Trump won’t release his tax returns, Pence said the Times story showed Trump was a businessman who “faced some pretty tough times 20 years ago.” He seemed to be referring to the period in the early 1990s when Trump’s casino business sank into bankruptcy. As part of an agreement with the banks that held his debt, Trump was put on a $450,000 monthly allowance.

The release of the partial returns have put Trump on the defensive after a week that began with Clinton scoring a victory in the first presidential debate and ended with Trump ranting in early morning tweets about Alicia Machado, the former Miss Universe he called fat more than a decade ago.

Clinton has been quick to say that Trump’s taxes are further proof that “he abuses his power, games the system, puts his own interests ahead of the country’s ... It’s always Trump first, and everyone else last.”

Trump’s campaign has neither confirmed nor contested the veracity of the documents published by the Times. Instead, the Trump campaign offered the mutually exclusive claims that the “alleged tax document” had been “illegally obtained” because Trump had not authorized its release. The campaign also said Trump was a successful businessman who would know how to fix U.S. tax laws.

Editor’s note: Donald Trump regularly incites political violence and is a serial liar, rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.