But Republicans argued the proposals had no place in the reconciliation bill.
“After decades of failing to enact their amnesty agenda, Democrats tried this latest, unprecedented gambit,” said Senator Mitch McConnell of Kentucky, the top Republican. “It was inappropriate, and I’m glad it failed.”
Many immigration activists refused to accept defeat.
“It’s an important moment for our country,” said Kerri Talbot, the deputy director of the Immigration Hub. “We’ve been trying to work with Republicans for many years, but they have failed to come to the table, and so we see this as an opportunity to go ahead and address some of the issues that have been waiting to be addressed for decades.”
Activists urged supporters to join a march on Tuesday, led by CASA and the Service Employees International Union, with stops outside the Washington, D.C., headquarters of Immigration and Customs Enforcement and the Capitol, to call for a path to citizenship for millions of undocumented migrants.
The parliamentarian is a little-known but crucial figure in the life of the Senate, which is largely governed by precedents and arcane rules that are subject to interpretation. The post becomes particularly important when it comes to reconciliation, which is supposed to be limited to provisions that have a direct impact on federal spending or revenues.
A nonpartisan career official who has worked in the parliamentarian’s office since 1999, Ms. MacDonough has heard detailed arguments from both sides in closed-door meetings on the immigration proposal.
Ms. MacDonough’s decisions are merely advisory, but several Democratic senators have indicated they would be reluctant to overrule her, and it is not clear that a majority would support doing so to win adoption of the immigration plan. She did not immediately respond to a request for comment.
Mr. Menendez pushed back against calls for her ouster, saying they were “not constructive.”
The budgetary cost of the changes in immigration law — which affect health care benefits, Medicaid spending and tax credits — exceeds $139 billion over 10 years, according to preliminary figures from the Congressional Budget Office. Democrats estimate the legalization push would add $1.5 trillion to the U.S. economy over the next decade, creating more than 400,000 jobs.