Washington (CNN) A Senate appropriations committee heard from nine Commerce Department bureau heads on Tuesday -- but not from Secretary Wilbur Ross, who declined to appear before the panel to answer questions about his budget request for 2020.

Ross' planned absence sparked consternation among Democratic senators who had planned to ask him about his controversial decision to include a citizenship question on the upcoming 2020 census.

It comes as House Democrats voted to subpoena Ross for key documents related to the census question.

That move was another step in the escalating war between newly empowered House Democrats and the White House, which has been reluctant to comply with requests for administration officials to testify before Congress.

"The Department believes that the rush to issue a subpoena is premature," Michael Platt, the assistant secretary for legislative affairs at the Commerce Department, wrote in a letter to Cummings obtained by CNN.

Ross faced Democrats on the oversight panel last month but they say he left additional questions unanswered.

"The committee is simply trying to determine the real reason Secretary Ross added the citizenship question," Oversight Committee Chairman Rep. Elijah Cummings said Tuesday. "The documents and the testimony covered by these subpoenas are critical to answering that question."

Ross is expected to appear before the equivalent House appropriations subcommittee on Wednesday afternoon, but that could change. Platt's letter to Cummings noted the same officials who will testify in the Senate on Tuesday intend to appear in the House as well, differing with the original witness list.

"The subcommittee plans to do its job by holding this hearing, and we expect Secretary Ross to likewise do his job by coming and testifying, as all secretaries of Commerce have been doing before the Appropriations Committee for decades," Rep. Jose Serrano, chairman of the House subcommittee, said in a statement provided to CNN.

Senate Democrats have expressed displeasure at Ross' absence. Other administration officials, including Treasury Secretary Steven Mnuchin and Health and Human Services Secretary Alex Azar , have declined to appear at House hearings since Democrats took control earlier this year.

In a statement last week, Vermont Sen. Patrick Leahy called Ross's decision not to attend the Senate hearing "a shame."

"I was looking forward to asking him why he misled me during his last appearance, one year ago, when he asserted that the Justice Department was 'the one who made the request' to include a controversial citizenship question on the Census," said Leahy, the top Democrat on the appropriations committee, which oversees the Commerce Department's funding.

The citizenship question has been blocked by two federal judges, and the Supreme Court is expected to rule on the matter in the coming months.

Opponents argue polling citizenship could depress census participation rates among noncitizens, resulting in an undercount in areas with high immigrant populations and skewing congressional representation among the states.

President Donald Trump tweeted Monday that the census would be "meaningless and a waste" if it did not ask if respondents were citizens.

Ross initially told Congress he made the decision at the behest of the Justice Department. But court documents indicate he considered adding the citizenship question to the census before he received the Justice Department's memo requesting the addition, and that Ross also communicated about the subject with former White House strategist Steve Bannon and another White House adviser beforehand.

During Ross's Oversight appearance last month, Democrats sought to determine whether his motivations went beyond his official explanation that the citizenship question is necessary to enforce the Voting Rights Act. Republicans on the committee cited past censuses when the question was included — most recently in 1950 — arguing Democrats were misrepresenting the issue.

"For the life of me, I do not know why the Democrats don't want to know how many citizens are in the United States of America," Ohio Rep. Jim Jordan, the top Republican on the panel, said during the March 14 hearing.

Some Democrats pointed to a May 2017 email in which Ross told aides he was "mystified" that action hadn't taken place on his request to include the question.

"We need to work with Justice to get them to request that citizenship be added back as a census question," he wrote at the time.

"You lied to Congress, you misled the American people, and you are complicit in the Trump administration's intent to suppress the growing political power of the non-white population," Missouri Democratic Rep. William Lacy Clay said at the time, calling on Ross to resign.

The Democrats' subpoenas would target Ross's emails about the citizenship question, as well as related DOJ documents and communications with outside entities including the White House, the Commerce Department, the Republican National Committee, the Trump campaign, and members of Congress.