Democrats would need at least five Republican votes to end a filibuster, a goal that has started to appear increasingly difficult to achieve. “Last week, I was pretty confident that, despite [Hagel’s] horrible showing at the confirmation hearing, he would still probably get to 60 votes,” a GOP Senate aide told National Review Online. “But I think his hiding the ball on [disclosing past associations] is going to hurt his ability get those five Republican votes.”

Hagel supporters have charged that filibustering a cabinet nominee would be unprecedented, but that simply is not true. In 2006, for example, Democrats filibustered the nomination of Dirk Kempthorne, President George W. Bush’s choice for secretary of the interior.

There is nothing Republicans can do to prevent Hagel’s nomination from passing out of the committee, which Democrats control. However, Senate sources confirm that several GOP members of the committee are mulling a walkout of Tuesday’s vote in protest, although Senator John McCain (R., Ariz.) has argued against the idea, calling it “disrespectful to Chairman Levin and at odds with the best traditions of the Senate Armed Services Committee.”