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Commerce Secretary Wilbur Ross will face Congress on Thursday to answer questions about his decision to add a citizenship question to the 2020 census.

His long-awaited appearance before the House Oversight Committee comes only days after a second federal judge said Ross violated federal law and the Constitution by hastily adding the question to the upcoming survey.

Democrats have sought answers from the Trump administration for more than a year about its handling of the all-important headcount, zeroing in not only on potential issues related to the citizenship question, but also why the agency is lagging in critical areas as it tries to prepare for the census. Now that the Democrats control the House, they've demanded the secretary's account.

In a letter obtained by NBC News, Ross said he was "disappointed" the committee did not honor his request to postpone the hearing to April, but said that the Department of Commerce, which oversees the Census Bureau, handed over almost 9,000 pages of documents regarding the census, among other matters, to the committee.

Ross' testimony Thursday could have legal implications as the U.S. Supreme Court plans to take up the battle over the citizenship question next month before its term ends in late June.

Here's what you need to know ahead of the hearing.

What will lawmakers ask?

Expect to hear a number of pointed questions, mainly from Democrats, about Ross' rationale for adding the citizenship question.

“Everything about adding the question doesn't pass the smell test," Rep. Jimmy Gomez, D-Calif., who sits on the committee, said. "I want to really get to the bottom of that."

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Last March, Ross told the House Ways and Means Committee under oath that the question was added at his direction after he received a request from the Department of Justice in December 2017 that said the data was needed to properly enforce civil rights laws.

However, documents released as a part of a multistate lawsuit showed that Ross inquired about adding the question much earlier. An agency spokesperson later told The Associated Press that discussions about it had begun earlier and nothing in the court records changes "the sound rationale" that Ross cited in his congressional testimony.

"I think that their rationale that they want to enforce the Voting Rights Act is just b.s.," Gomez said. "I would like to see the committee focus on that line of question. Did he actually lie to Congress?”

He added, “We’re going to hold him accountable for what he said.”

Rep. Jackie Speier, D-Calif., agreed.

"The hearing will further show that Secretary Ross’ voting rights justification was just a cynical pretext to trample on the rights of immigrants," she said in a statement. "My home state of California, with its large and vibrant immigrant communities, has a lot at stake: the 2020 census’ lack of readiness and the alienating effect of the citizenship question threaten to undercount California’s population by an estimated 1.6 million and costing the state more than $1.5 billion in federal resources."

Also, expect to hear questions about the preparedness of the Census Bureau. If the bureau does not meet its goal of addressing issues like privacy concerns and hiring shortfalls, among others, that could also result in a undercount.

What's the latest on the census lawsuits?

Last year, a coalition of 17 states, Washington, D.C., and six cities sued the Commerce Department, citing a concern that fewer immigrants will respond to the survey and therefore decrease the accuracy of the 2020 census, which determines funding allocation and how political districts are drawn. More groups also sued to block the question.

In January, a New York federal judge blocked the administration of President Donald Trump from including the census question. The judge ruled that Ross "violated the public trust" by adding it, calling his decision "arbitrary and capricious."

Then, earlier this month, a second federal judge in California blocked the question again, arguing that it violated administrative law and was unconstitutional because it prevents the government from conducting an accurate count of the population.

"In short, the inclusion of the citizenship question on the 2020 census threatens the very foundation of our democratic system — and does so based on a self-defeating rationale," the judge wrote.

What other issues does the census face?

Uncertainty surrounding the Trump administration's plans to add the citizenship question is only one of a number of critical issues plaguing the census bureau.

Through a series of reports last year, the Government Accountability Office, a watchdog agency, detailed a range of issues facing the census, including budget shortfalls, potential cybersecurity weaknesses, hiring shortages, testing cutbacks, and a printing company hired by the Census Bureau going bankrupt — all of which could impede the accuracy of the agency's constitutional mandate.

Since then, the bureau has awarded a $114 million contract to a new printing company and claimed it is on-track to address the issues identified by the GAO and complete its work on time.

What is the timeline for the census?

The bureau plans to begin its operations this spring, including printing postcards, sending information on how to take the census online for the first time and hiring enumerators, among other things. The bureau must count the population by April 1, 2020, deliver state counts to Trump by Dec. 31, 2020, and provide redistricting data to the states by spring 2021.