The US Congress has strong arguments for obtaining President Donald Trump’s tax returns, but first faces a long court battle that could extend into the 2020 presidential election, some legal experts said.

Democratic Party leaders, who took control of the House in November, have recently stepped up pressure on Trump to release tax records from 2013 to 2018, which legal experts said could shed light on the president’s business dealings.

Richard Neal, chairman of the House Ways and Means committee, which oversees tax matters, has given the Internal Revenue Service until April 23 to produce the documents.

Trump’s personal lawyer, William Consovoy, in a letter to the Treasury Department on Monday, accused Neal’s committee of using its “investigatory tools to unlawfully retaliate against a political opponent”.

Lawmakers can go to court and ask a judge to force disclosure of the tax returns, but such a process could unfold slowly and become an issue in the 2020 election, legal experts said.

“I expect Trump will stall at every opportunity,” said Steven Rosenthal, a tax lawyer with the Urban-Brookings Tax Policy Center in Washington, adding that “delay is a victory for Trump”.

Consovoy did not immediately respond to a request for comment.

A law from 1924 says that the IRS “shall furnish” any tax return requested by the chair of the Ways and Means committee.

“If you look at the plain language of the statute, Neal is on strong footing here,” said Jessica Levinson, a professor at Loyola Law School in Los Angeles.

But this does not mean that Congress has unlimited power to request tax returns, experts said.

If Congress invokes this statute, it must be acting with a legitimate legislative purpose, legal experts said, citing a 1957 ruling by the US Supreme Court.

In that decision, the court said that Congress was not a “law enforcement agency” that can seek information to uncover crimes.

Neal said in an April 4 statement that the documents will aid Congress in evaluating whether the IRS has been effective in enforcing tax laws against the president.

“The IRS has a policy of auditing the tax returns of all sitting presidents and vice presidents, yet little is known about the effectiveness of the law,” Neal said.

Consovoy in Monday’s letter said that “no one actually believes” Neal’s “invented” reasons for wanting the tax returns.

Ross Garber, a lawyer in Washington focusing on political investigations, called Consovoy’s argument “plausible” and said it was possible a court would side with the president.

Garber said Congress’ authority to request the documents would be stronger if it were conducting impeachment proceedings. Courts have held that Congress’ investigative powers are broader in the impeachment context, he said.

But Edward Kleinbard, a tax law professor at the University of Southern California, said Neal’s stated reason for demanding the tax returns — ensuring that tax laws are being enforced against Trump — was more than sufficient.

“The integrity of tax system depends on peoples’ willingness to assess tax against themselves. The president of the US is the taxpayer-in-chief, and he sets the standard,” Kleinbard.

Kleinbard, who formerly served as chief of staff to Congress’ Joint Committee on Taxation, noted that Trump was the first US president in nearly 40 years who has not voluntarily released his tax returns to the public.

Trump has said he cannot make the returns public because they are under audit.

Kleinbard called that argument “nonsense,” saying nothing prohibits a person under audit from releasing his tax returns.