(CNN) Commerce Secretary Wilbur Ross said Thursday that his congressional testimony on adding a citizenship question to the 2020 census has been truthful, even displaying a series of photographs in rebuttal to Democrats who argue otherwise.

"I testified truthfully to the best of my ability in response to what my understanding of what the questions were," Ross told the House Oversight Committee on Thursday.

Democrats have claimed that Ross misled them at a March 2018 hearing, when he appeared to testify that he had not discussed with the White House whether to add the controversial question, which many fear could prompt even legal immigrants to avoid completing the census -- leading to an inaccurate count of the US population that would impact a wide range of social services and other government programs.

"Has the President or anyone in the White House discussed with you or anyone on your team about adding this citizen question?" asked New York Democratic Rep. Grace Meng at that 2018 hearing.

"I'm not aware of any such," Ross replied.

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