Ben Nelson, the conservative Democratic Senator who in recent months has had a voting profile not unlike a conservative Republican, announced his opposition to the DREAM Act, leaving Democratic leaders in search of additional Republican votes to make up the difference.

The DREAM Act would allow undocumented students who were brought to this country as children a meaningful path to citizenship if they entered college or military service. Oddly, Nelson’s justification was that he wouldn’t support anything that “adds to the military.”

“I’m not going to support any act that I don’t think adds to jobs, or military or to the economy. Consequently I won’t support any motion to proceed or any kind of cloture on the DREAM Act,” Sen. Ben Nelson (D-Neb.) told POLITICO. “In addition, I think that has to be part of an overall comprehensive solution to immigration once we have the border secured, not until then.”

The Defense Department’s own FY2010-2012 Strategic Plan cited the DREAM Act as an important measure to bolster an all-volunteer military force. So even by Nelson’s own standards, he is wrong.

What’s more, allowing undocumented students who didn’t make the decision to come to America to get citizenship has a clear economic benefit. These students would be able to get higher-paying jobs and contribute to the economy in a much more important way than if they were consigned to the underground economy as an undocumented worker. High-skill immigrants bring multiple benefits to the US economy, and we should be welcoming more of them, not placing barriers in their way.

As for the raw vote totals, Nelson’s opposition means that only 57 Democrats would be available for passage, if the rest of them stayed firm (an open question). While Richard Lugar and Bob Bennett have publicly announced their support for a standalone bill on the Republican side, others have hemmed and hawed. It will be difficult to round up the votes to break an expected filibuster, especially if more Democrats defect.