The Senate parliamentarian determined that lawmakers cannot include a pathway to citizenship for illegal aliens in a budget reconciliation package, but Rep. Ilhan Omar, D-Minn., is pushing the Senate and the Biden administration to do it anyway.

Senate Parliamentarian Elizabeth MacDonough determined Sunday night that the provision fell outside the scope of what is allowed in a reconciliation bill – which cannot be filibustered, thus requiring the support of only a simple majority in the Senate.

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"This ruling by the parliamentarian, is only a recommendation. @SenSchumer and the @WhiteHouse can and should ignore it," Omar said. "We can’t miss this once in a lifetime opportunity to do the right thing."

MacDonough said the provision was "a policy change that substantially outweighs the budgetary impact of that change," as it would change the immigration status of 8 million people. She also said it would raise the deficit by an estimated $140 billion over 10 years.

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The parliamentarian also noted that if a provision granting a new immigration status to people could be done through reconciliation, so could a provision stripping such status.

"[P]ermitting this provision in reconciliation would set a precedent that could be used to argue that rescinding any immigration status from anyone - not just those who obtain LPR status by virtue of this provision -- would be permissible because the policy of stripping status from any immigrant does not vastly outweigh whatever budgetary impact there might be,"  MacDonough said.

The White House took the parliamentarian’s decision more seriously but continues to support efforts in the Senate to include an immigration measure in the bill.

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"The President has made very clear that he supports efforts by Congress to include a pathway to citizenship in the reconciliation package and is grateful to Congressional leadership for all of the work they are doing to make this a reality," said a White House spokesperson. "The Parliamentarian’s ruling is deeply disappointing but we fully expect our partners in the Senate to come back with alternative proposals for the Parliamentarian to consider."

Schumer said he was "deeply disappointed" by MacDonough’s decision and said, "Senate Democrats have prepared alternate proposals and will be holding additional meetings with the Senate parliamentarian in the coming days."

Fox News' Chad Pergram contributed to this report.