The US Senate votes Tuesday on whether to advance a controversial Republican bill that funds homeland security but rolls back President Barack Obama's immigration plan, with Democrats revolting against the measure.
In December, lawmakers funded all federal departments through the end of fiscal year 2015 except for the Department of Homeland Security, which it funded only through February 27 so the new Republican-controlled Congress could put brakes on Obama's plan to shield millions of undocumented workers from deportation.
The subsequent battle over DHS funding has become one of the most turbulent congressional debates of 2015, with Republicans irate over what they describe as presidential overreach, and most Democrats lining up behind Obama, who has threatened to veto the legislation.
Last month, the House passed the bill funding DHS through September, but attached five riders that would strip Obama's authority to carry out his unilateral immigration action.
Should the Senate fail to follow suit, lawmakers will be faced with forging a compromise in barely three weeks, or see parts of the department that oversees border security, cyberterrorism prevention and the US Secret Service which protects the president go into partial shutdown.
Senate Majority Leader Mitch McConnell acknowledged Tuesday's vote was largely about challenging Democrats and their support for the president while he "repeatedly reached beyond his authority."
"The truth is the latest power grab isn't really about immigration reform; it's about making an already-broken system even more broken," McConnell added.
"The question is, do Democrats agree with the president? Well, we'll soon find out."
Republicans, who hold 54 of the Senate's 100 seats, need 60 votes to advance the measure.
Democrats oppose the bill's "poison pill" immigration inserts, and insist on passing a clean DHS funding bill.
"Take off the riders, take off the political extraneous things," number two Senate Democrat Dick Durbin said.
"It is incredible to me that we have refused to provide the funds that the DHS needs to keep America safe."
The Democratic Senate caucus was meeting Tuesday before the roll call to discuss how to vote on the measure.