Hillary Clinton and her Democratic allies are stepping up their battle with Donald Trump over his tax returns, launching a new attack ad blasting the Republican nominee for not releasing the documents while pushing a pointed piece of legislation that would require presidential nominees to make their returns public.

The Republican nominee, in declining to release his returns, has said he cannot provide them because he’s being audited by the IRS. 

"There's nothing to learn from them," he also told the Associated Press in May.

Trump has tried to brush off the calls, increasingly turning to security and immigration issues on the campaign trail -- but Clinton and the Democrats have focused sharply on Trump’s taxes, pushing speculation that they may show links to Russian oligarchs or some other political damaging detail.

The new Clinton TV ad begins with a dated, pre-campaign clip of Trump promising to release his returns in a presidential race – and goes on to air more recent clips of Trump’s Republican critics, like Mitt Romney, suggesting the candidate has not followed through on that pledge because there’s a “bombshell” in the documents.

The ad asks, “What is Trump hiding?”

Two Clinton allies, Sens. Ron Wyden, D-Ore., and Chris Murphy, D-Conn., also plan to renew their push Thursday for legislation known as the Presidential Tax Transparency Act.

Wyden initially introduced the bill in May. It would require a candidate to release their most recent three years of tax returns within 15 days of becoming a party presidential nominee.

In a striking provision, the bill would direct the Treasury secretary to provide the returns to the Federal Election Commission for public release if the candidates do not comply.

Trump, meanwhile, is expected to start airing his first general election ads in battleground states starting Friday. The campaign confirmed Thursday it has purchased a $5 million ad buy over a 10-day period in battleground states: Florida, Virginia, Pennsylvania, Ohio and North Carolina.

The move comes after he shook up his campaign, installing a new campaign manager and chief executive, while turning up the heat on the Obama administration and Clinton for their record in the war on terror.

At a Fox News town hall with Sean Hannity, Trump accused the administration of waging a “politically correct” war against ISIS and warned that more terror attacks will take place.

 

"We're not taking it to them," Trump said. "And we have to take it to them."

"We can be nice about it, say it's never going to happen again, there will be more [attacks]," Trump later added. "What we're doing by allowing tens of thousands of people in ... we don't know anything about, it's going to happen again because there's something wrong, and until we figure it out, we have to stop it."