In what could be one of his most politically significant final acts as president, Donald Trump plans to exclude millions of undocumented immigrants from the official 2020 Census figures used to allocate political power and billions of dollars in federal funds. But first, the U.S. Supreme Court has to sign off. The justices on Monday will hear oral arguments over Trump's effort -- already twice rejected by lower federal courts -- that would break from more than a century of precedent in determining apportionment of the 435 congressional districts across all 50 states. If successful, it would boost the influence of predominantly conservative, Republican states and rural communities while drawing resources away from more liberal, Democratic states and urban areas. The Constitution requires an "actual enumeration" be performed every 10 years to account for changes in population and that decennial redistricting be based on "the whole number of persons in each state," regardless of citizenship or immigration status. Since taking office, Trump has sought to minimize the influence of non-citizens in American politics by sidelining them from the count. After the Supreme Court in 2019 blocked his attempt to add an explicit citizenship question to the census form, Trump directed the Census Bureau to rely on existing government data to derive a total of undocumented immigrants in each state -- and then subtract that figure from the overall sum. "For the purpose of the reapportionment of representatives following the 2020 census, it is the policy of the United States to exclude from the apportionment base aliens who are not in a lawful immigration status," Trump wrote in a July memorandum. MORE: Justices rule courts cannot police partisan gerrymandering A coalition of 20 states, led by New York, 10 cities, five counties and a handful immigrant advocacy groups immediately challenged the move. They called Trump's plan a clear violation of the Constitution and an "arbitrary and capricious" decision that breached federal law governing administrative procedures. In legal briefs, the parties cite an "unbroken historical and legislative practice" spanning more than 200 years of the Census Bureau counting "millions of undocumented immigrants who have lived here for decades, intend to remain and will in fact stay ... (as) usual residents under traditional criteria." In September, a three-judge panel in New York sided with the states against Trump, calling the issue "not particularly close or complicated." Last month, a separate panel of judges in California also dealt a blow to the administration, saying "It seeks to do what Congress has not authorized and what the President does not have the power to do." An estimated 10.5 million immigrants living in the U.S. do not have legal status, according to a Pew Research Center analysis of U.S. Census Bureau data. And more.