WASHINGTON (Reuters) - The top Republican on tax policy in the U.S. Senate said on Wednesday that he will seek President Donald Trump's tax returns if Democrats in the House of Representatives obtain them.

Senator Chuck Grassley, chairman of the Senate Finance Committee, told reporters that he has no particular interest in requesting the documents from the Treasury Department and does not want to see the Internal Revenue Service used for political purposes.

"If the House of Representatives are going to get them, then I want the Senate Finance Committee to have them," said Grassley, who as the Finance chairman is the only lawmaker in the Senate authorized to request confidential tax records from U.S. Secretary Steven Mnuchin.

"I don't want what ... the House Democrats are using the IRS for - for political purposes," he added. "If they get them, we'll just ask for them."

A Grassley spokesman said the Iowa Republican believes that both chambers should have access to Trump's returns as a way of maintaining proper checks and balances.

Federal law authorizes the chairmen of the Senate Finance Committee, the House Ways and Means Committee and the nonpartisan congressional Joint Committee on Taxation to seek confidential tax information from the Treasury.

Representative Richard Neal, the Democratic Ways and Means chairman, is preparing to request Trump's returns as part of an oversight effort by Democrats to determine whether the president has complied with U.S. tax laws or has conflicts of interest stemming from his global business empire. Democrats want to release the documents to the public.

However, Grassley believes individual tax returns obtained by Congress should be kept confidential and used to inform legislation, not for oversight, his spokesman said.

Democrats took control of the House last year in an election marked by voter opposition to Trump. The Senate remains under Republican leadership.

As a presidential candidate in 2016, Trump broke with decades of precedent by refusing to release his tax returns. He has continued to keep them under wraps as president, saying they are under audit by the IRS.

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But in congressional testimony last week, Trump's former personal lawyer Michael Cohen said he does not believe Trump faces an IRS audit and said the president refused to release his returns because he feared it would lead to an audit and tax penalties.

Cohen also testified that Trump has altered the value of some of his property holdings to obtain lower tax bills and once called the federal government stupid for giving him a $10 million tax refund after he slashed the salaries of his workers.

(Reporting by David Morgan in Washington; Editing by James Dalgleish)