Democrats won’t be able to use their $3.5 trillion spending package to pave the way to citizenship for millions of immigrants, the Senate’s parliamentarian said late Sunday. Elizabeth MacDonough, who as parliamentarian interprets the Senate’s rules, wrote: “Changing the law to clear the way to [legal permanent resident] status is tremendous and enduring policy change that dwarfs [any potential] budgetary impact.”

The ruling comes as a crushing setback to Democrats looking to craft major immigration reform. It closes off a pathway to permanent residency or citizenship for, by some estimates, around 6 million people, including children who were brought to the U.S. illegally, immigrants fleeing countries wracked by violence or natural disasters, essential workers, and farm workers.

Democrats and immigrants’ rights groups have dug in their heels and promised to fight the decision. “The parliamentarian’s recommendation is just that: a recommendation,” tweeted Greisa Martinez Rosas, executive director of United We Dream, an immigrants’ rights group. “Democrats can still deliver citizenship this year. It’s time they side with millions of immigrants, our friends, and our communities. This isn’t over. We need citizenship THIS YEAR.”