Donald Trump has asserted executive privilege over subpoenaed census documents, as Democrats prepare to vote on whether to hold his attorney general in contempt.

As the House oversight committee voted on whether to hold attorney general William Barr and commerce secretary Wilbur Ross in contempt of congress over their alleged effort to politicise the 2020 census, the department of justice said the president had sought to assert executive privilege over the material being sought by Democrats.

Shortly before the committee was due to vote, the department of justice told its Democratic chairman, Elijah Cummings, the administration was “engaged in good-faith efforts” to satisfy the committee's oversight needs and said the contempt vote was premature.

“Unfortunately, rather than allowing the department to complete its document production, you have chosen to go forward with an unnecessary and premature contempt vote,” assistant attorney general Stephen Boyd wrote in Wednesday’s letter.

The Trump administration has sought to add a citizenship question to the census, something it says will help provide a better understanding of where citizens live. Critics say it could put off minorities from responding.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. 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Mr Cummings’s committee has said Mr Barr and Mr Ross, whose department oversees the census, have been stonewalling their request for information.

It is voting to hold Mr Ross in contempt for failing to turn over documents requested through a subpoena, and a similar censure for the attorney general.

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“Both secretary Ross and attorney general Barr are refusing to comply with duly authorised subpoenas from Congress,” Mr Cummings said on Monday.

“Because they are in contempt of Congress, on Wednesday, the committee will vote to move forward to enforce our bipartisan subpoenas.”

Senior White House official Kellyanne Conway defended the president’s decision, the latest part of strategy of denials and non-cooperation Mr Trump has ordered as Democrats seek to obtain either documents, or witness testimony as they pursue a series of probes.

“The president, the department of justice, has every right to do that,” she told MSNBC.

“They're asking for documents that are privileged and I would hope that they can continue to negotiate and speak about what is appropriate and what is not, but the world is watching. This country sees that they'd rather continue to investigate than legislate.”

Holding an official in contempt of Congress, is a rare step and is seen as a censure by the legislative branch of somebody from another branch of government.

Democrats could also ask the courts to enforce a congressional subpoena, already issued. In theory, it could also lead to criminal prosecution.