A Senate official dashed Democrats' hopes of expanding citizenship for migrants in the $3.5 trillion budget bill

• The Senate parliamentarian said immigration reform can't be part of the $3.5 trillion bill.
• She ruled it is incompatible with the "reconciliation" mechanism being used to pass it without the GOP.
• The decision effectively means that any moves to reform the system will need Republican support.

A Senate official on Sunday ruled that Democrats will not be able to include immigration reform measures in their sweeping $3.5 trillion stimulus bill.

The decision, by Senate parliamentarian Elizabeth MacDonough, is a harsh blow to Democrats hoping to squeeze changes to the system past with their tiny Congressional majority.

The Biden administration and Democrats had hoped to ease immigration rules and provide a path to citizenship for millions of migrants, a plan put in jeopardy by MacDonough's decision.

She was able to make the ruling because she oversees the application of the so-called "budget reconciliation" process, which allows certain financial measures to pass by a smaller margin than usual.

Without that loophole, Democrats would need to find support from Senate Republicans for their plans, an unlikely prospect.

MacDonough said: "Changing the law to clear the way to [legal permanent resident] status is tremendous and enduring policy change that dwarfs [any potential] budgetary impact," said MacDonough in documents explaining the decision, reported the Associated Press.

If Democrats decide to try and pass the immigration reform as a conventional Senate bill, it'd require 60 votes to pass and is all but certain to be blocked by Republicans.

Progressives and Democrats have long sought to provide a path to citizenship for migrants from certain key groups who currently inhabit a sort of legal limbo.

They include so-called "Dreamers" who were brought to the US as children, have lived in the US for their entire lives, but have no formal citizenship.

Other groups affected include some refugees allowed into the US because they were fleeing disasters or other catastrophes, and essential workers for sectors such as agriculture are others.

"We are deeply disappointed in this decision but the fight to provide lawful status for immigrants in budget reconciliation continues," Senate Majority Leader Chuck Schumer said in a statement.

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

Senate Minority Leader Mitch McConnell attacked the Democrats, saying: "Democratic leaders refused to resist their progressive base and stand up for the rule of law, even though our border has never been less secure."

He said that attempting to pass the reforms using budget reconciliation were illegal.

The overall bill is President Joe Biden's signature piece of legislation, and will aim to fund a wide range of economic, green energy, infrastructure and social care reforms. Democrats are aiming to pass the bill over the next few weeks.