US House of Democrats issued subpoenas against top government officials demanding President Donald Trump’s tax returns, the latest twist in the sharpening battle between the White House and opponents in Congress.

House Ways and Means Committee chairperson Richard Neal said he subpoenaed Treasury Secretary Steven Mnuchin and Internal Revenue Service commissioner Charles Rettig after they missed two congressional deadlines to turn over six years of the president’s personal and business tax data.

Trump broke decades of precedent by refusing to publish his tax returns as a candidate in 2016, or since he has been president, claiming he could not do so as he was under audit.

As the majority party in the House of Representatives, Democrats have broad investigative powers.

They have begun issuing subpoenas on multiple fronts as part of their investigations into Russia’s interference in the US election and possible obstruction of justice by the president.

Neal said he requested the financial records as part of his responsibility for conducting oversight of the tax system, and as related to the wide-ranging investigation into Russian interference in the 2016 US elections.

“While I do not take this step lightly, I believe this action gives us the best opportunity to succeed and obtain the requested material,” Neal said in a statement.

Republicans have blasted the request as politically motivated, with Mnuchin expressing concern that Democrats’ true purpose was to publish the president’s tax returns.

The request is expected to be ignored, after Trump said he was using his executive privilege to prevent the handover to Congress of any material connected to the Russia probe.

Lack of compliance would almost certainly trigger a court battle that could drag on for several months, as Trump and the Democrats gear up for a 2020 election fight.