(CNN) President Donald Trump put a political spin on the US census Monday when he tweeted that it would be "meaningless" if people aren't asked whether they are citizens -- even though many experts fear the question would drive down the official national population count next year.

As it happens, that less accurate count could help his Republican Party in the reapportionment of congressional seats and electoral votes after 2020.

Officials at the Census Bureau, meanwhile, said Monday that they had prepared two versions of the census -- one with the citizenship question and one without -- while they wait for the US Supreme Court to decide if it can be asked. Lower courts have sided with states that want the question kept off the 2020 census. The last time it was asked on the full census was back in 1950, which raises the question of whether the US President thinks the last six census efforts were meaningless.

Can you believe that the Radical Left Democrats want to do our new and very important Census Report without the all important Citizenship Question. Report would be meaningless and a waste of the $Billions (ridiculous) that it costs to put together!

"Can you believe that the Radical Left Democrats want to do our new and very important Census Report without the all important Citizenship Question," Trump tweeted. "Report would be meaningless and a waste of the $Billions (ridiculous) that it costs to put together!"

He didn't mention the official argument his administration had made in federal court: that the question is necessary to properly enforce the Voting Rights Act.

Gains and losses

The stakes for the census are extremely high. The main role of the population figure obtained is determining reapportionment of the 435 seats in the US House of Representatives, and by extension, electoral votes in presidential elections. Each state gets at least one seat and the other 385 are distributed based on population.

After the 2020 census, Rust Belt states are expected to lose seats and electoral votes, while states with growing populations like Texas, Florida, Colorado and North Carolina could gain. And those numbers will be locked for a decade.

Undercounting is worst among minorities and immigrants -- traditional constituencies of Democrats.

That's one reason the Trump administration's dogged effort to include the citizenship question alarms statisticians and census experts: They have not tested how adding the question will affect the count.

"Do we know directly how adding a citizenship question would affect participation in the census and therefore the final count? The answer is no," said Terri Ann Lowenthal, a former staff director of the House census oversight subcommittee. "And that's why every expert in the field and six former census directors counseled against adding it without testing."

Former census directors Robert Groves and Steven Murdock argued in the journal Science last year against adding untested questions such as the citizenship question late in the process: "Mistaking the decennial census as only an act of politics, solely useful for updating the distribution of political power, threatens the scientific basis of its credibility."

The Census Bureau has added a test of the citizenship question this summer, but the results will only help determine how to get the largest number of responses and how it will train census workers -- not whether to include the question or not.

Fears of a miscount

Census Bureau researchers had raised the alarm in 2017 that the political climate was driving up distrust of the government, particularly as anti-immigrant rhetoric entered the US conversation. The census researchers wrote of immigrants in focus groups who intentionally gave false information because they feared the responses could be used against them, although sharing individual responses with law enforcement is illegal.

survey by the Census Bureau in 2018 also identified mistrust of the census and the government and suggested the citizenship question could be a barrier to response rates.

Despite these warnings, the Trump administration -- via Commerce Secretary Wilbur Ross, who oversees the Census Bureau -- decided to add the citizenship question, to better enforce the Voting Rights Act, Ross said

But the end result could be that the census fails to count a bunch of people, particularly in areas with large numbers of noncitizens or undocumented.

Democrats in the House of Representatives could move Tuesday to subpoena documents and testimony to challenge whether the Voting Rights Act is the real reason Ross and Republicans added the question.

How else is the data used?

The official count of people in the country is used not just to determine how many lawmakers each state gets in Congress and how many electoral votes they get in presidential elections. Census data today also is used to determine how more than $675 billion in federal funding is allocated to states, according to the bureau.

States use the data too, which means it has implications from the federal to the local level.

"Census numbers are used not only for congressional apportionment, but to allocate fair political representation in state legislatures, city councils, all the way down to school boards, and to distribute government resources where they are most needed," said Lowenthal.

Citizenship question asked in separate survey

The government has been asking the citizenship question on a separate survey, the American Community Survey, also conducted by the Census Bureau, that determines the number of citizens versus noncitizens along with a bunch of other data, everything from how far a person commutes to work to what language they speak to whether they have indoor plumbing. That type of survey, which relies on a representative sample and not a specific count, cannot by law be used for the population count.

While the American Community Survey uses the sample to give an in-depth snapshot of American society and is conducted every year, the census asks fewer than 10 questions, seeks to ask them of every American and is taken only every 10 years. And you're required by law to respond to the census, although the response rate in 2010 was only about 74%. Census workers try to speak directly to people from the households -- 47 million of them in 2010! -- that don't mail in their questionnaires.

The 2020 census will be the first where most Americans submit their responses online, which is creating a new layer of security concerns

'All persons' should be counted

Who to count and how has been a question since the country was founded. The Constitution originally counted "free persons" while excluding some American Indians and counting slaves as a fraction of a person. Until 1840, slaves were counted under their owners' names , according to the National Archives.

The stain of that compromise on the Constitution -- that slaves were not counted as entire people -- was fixed by the 14th Amendment, which included the language: "Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed."

The count to be used, then, is not for citizens but rather for "all persons."