OBSTRUCTIONIST NELSON…. The Senate Democratic caucus has 59 members. Sen. Dick Lugar (R-Ind.) supports Dawn Johnsen’s nomination to head the Justice Department Office of Legal Counsel. So, what’s stopping the majority from breaking the Republican filibuster and confirming Johnsen? Ben Nelson is.

Sen. Ben Nelson’s opposition to President Obama’s choice to head the Justice Department Office of Legal Counsel appears to be the key obstacle to her confirmation. Democrats say Dawn Johnsen, an Indiana University law professor, has 59 backers in the Senate — just one vote shy of cloture. Nelson, a Nebraska Democrat, is standing firmly against her appointment, pointing to Johnsen’s job 15 years ago as a counsel to the abortion rights group NARAL.

All of which has left Nelson’s critics furious. Where was the principled opposition from the Senator during the Bush years? Why is he refusing to show Obama the same deference that he offered the previous administration?

It’s hardly news that Nelson is the most conservative Senate Democrat, but even for him, this is ridiculous. Just a few weeks ago, Nelson’s office assured observers, “the senator doesn’t support obstruction and usually votes for cloture,” even though he doesn’t support Johnsen.

Which is fine. Nelson has “concerns” about Johnsen because she, like most Americans and the vast majority of Democrats, is pro-choice. It stands to reason that he’d vote against her nomination.

But what’s at issue here is Republican obstructionism. Nelson isn’t just opposing a qualified nominee, he’s refusing to even let the Senate vote on her nomination. A senator who claims to be a Democrat will not let a Democratic Senate vote, up or down, on a Democratic president’s choice for the OLC. This from a man who claims to oppose “obstructionism.”

To add insult to injury, Nelson found filibusters against George W. Bush’s nominees to be offensive, and routinely voted with Republicans to cut off Democratic efforts. He explained that he believed Bush’s nominees “deserve an up-or-down vote,” even when the person in question “isn’t popular with the special-interest groups in Washington.”

Nelson, in other words, is going easier on Bush than Obama — all because Dawn Johnsen worked for NARAL 15 years ago.

Whenever you hear talk about Dems having a “filibuster-proof majority,” remember that Ben Nelson is one of the 60, and he’ll betray his allies whenever it suits his purposes.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.