"That's one of them. That's one of why we lost in part," the Democratic senator from Pennsylvania told Fox News.

The Senate appears poised to advance a bill that would expand the federal government's mandate to detain immigrants who are in the country illegally, with Republicans putting the issue front and center in the new Congress.

The measure easily cleared the 60-vote filibuster but marked the first of likely several procedural hurdles before Republicans can declare victory.