Who are the obstructionists now? For nearly five years, Democrats in Congress denounced the GOP as "the party of no" for routinely using the Senate's filibuster rule to block legislation and presidential nominations from passing with a simple majority vote. On Tuesday, it was the Democrats' turn to stand in the way.

All 46 senators in the new minority joined together to oppose a $40 billion House Republican spending bill for the Department of Homeland Security that sought to reverse President Obama's unilateral move to protect millions of undocumented immigrants from deportation. The measure received 51 votes, but that was well short of the 60 it needed to be eligible for debate in the Senate. Republicans have vowed to try again, but it's clear they will have to come up with another way to fund the department before a February 27th deadline.

It was an interesting tactical move for Democrats. They could have allowed the bill to pass, since it would have faced a certain veto from President Obama. Harry Reid, the majority-leader-turned-minority-leader, has said he doesn't want to mimic the obstructionism of Republicans that so frustrated him when he ran the Senate. Standing down on the homeland security bill would have been one way of showing that, but Democrats also knew that allowing Republicans to spend a week or two to debate and pass it could waste precious time before the department's funding expires. And keeping the bill mired in the Senate's procedural muck makes life more difficult for Mitch McConnell, the new majority leader, who would relish the chance to blame an Obama veto for the bill's failure rather than arcane voting rules. this way, the burden for finding a workable solution still resides with Republicans.

The homeland security debate has upended the usual dynamic in another way. It's hard to imagine the hawkish GOP of the George W. Bush era demanding conditions on any legislation related to national security. Now it's the Democrats who are accusing Republicans of "playing politics with national security," as both Reid and President Obama put it on Tuesday. Senator Ted Cruz countered that it was "the height of irresponsibility for Senate Democrats to filibuster DHS funding to protect President Obama's unconstitutional amnesty." But Republican leaders knew that the House bill would never be signed into law, and Speaker John Boehner is already preparing to authorize a lawsuit over the president's immigration move, a backup plan of sorts.

Republicans must now figure out how to keep the paychecks flowing for thousands of the nation's frontline security agents. (Most of the department wouldn't shut down if funding lapsed, since so many of its employees are considered essential, but their checks would be delayed.) Democrats are hoping the GOP will simply remove the immigration language altogether, but Republican leaders may try to find a token restriction on the president's policy as a way of saving face. If nothing else, the vote on Tuesday showed that for all of the Democrats' complaints about the GOP's procedural maneuvers when they were in charge, they are plenty willing to try them out now that the roles are reversed.