Senate Parliamentarian Elizabeth MacDonough on Sunday dealt a significant blow to Democrats’ hopes of including a pathway to citizenship for millions of illegal immigrants – ruling that they can’t include it in their $3.5 trillion reconciliation bill.

Democrats are seeking to pass the massive spending bill via the budget reconciliation process – which allows them to avoid a Senate filibuster and pass the legislation with just 50 votes. 

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As part of this bill, Democrats had sought to include a number of immigration provisions, including a pathway to legal permanent residence and citizenship for a number of categories of illegal immigrants. Those eligible included recipients of the Deferred Action for Childhood Arrivals (DACA) program, those who are protected by Temporary Protected Status, farmworkers, and those deemed "essential workers."

It was seen as Democrats’ best chance of passing pathways to citizenship for illegal immigrants, after efforts to pass a sweeping immigration reform bill earlier in the year stalled in the face of Republican opposition.  

Given the 50/50 split in the Senate, Democrats would have needed to get 10 Republican votes to overcome a filibuster, and Republicans have largely snubbed efforts to grant legal status to illegal immigrants – citing the effect they believe it would have on the already surging crisis at the southern border.

But in a preliminary ruling, MacDonough ruled that such language was inappropriate for a budget reconciliation bill.

MacDonough cited Congressional Budget Office estimates that 8 million people (nearly 7 million of whom are present illegally and currently ineligible for adjustment of status)  would be adjusted to legal permanent residence (LPR) status under the program, and that it would increase the deficit by $140 billion over 10 years due to the expansion of the social safety net and benefits programs it would require.

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She noted that the "essential" workers category covers 18 major categories and over 220 sub-categories of employment, and that there are waivers for many conditions of ineligibility.

In her ruling, MacDonough noted the number of non-fiscal impacts that such a change would have, concluding that such a change in the law is a "tremendous and enduring policy change that dwarfs its budgetary impact" and is therefore inappropriate for a reconciliation bill.

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Senate Majority Leader Chuck Schumer, D-N.Y., said in a statement that Democrats are "deeply disappointed" but pledged to continue fighting to provide such measures in the reconciliation process.

"Senate Democrats have prepared alternate proposals and will be holding additional meetings with the Senate parliamentarian in the coming days," he said.

The ruling will potentially cause problems if Democrats have to slash the total size of the legislation to gain the support of moderate Democrats. Analysts claimed that including amnesty provisions could help placate the liberal wing of the party if the bill was trimmed.

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Senate Minority Leader Mitch McConnell, R-Ky., had previously warned that Democrats had better not try to disregard the parliamentarian's ruling if it went against them.

"Abiding by the parliamentarian is essential to the function of the Senate," McConnell said. 

Fox News' Chad Pergram and Tyler Olson contributed to this report.