We told you yesterday about Sen. Robert Menendez's (D-NJ) frustration over provisions in the $410 billion 2009 spending bill that would loosen the trade embargo against Cuba. But while we're talking about Menendez's vote on that bill, we shouldn't forget that he is also holding up the confirmation of John Holdren and Jane Lubchenco, two nominees to become senior science advisers in the Obama administration.

And he's not alone. It seems that multiple anonymous senators are now holding up Holdren and Lubchenco's nominations, as Senate Commerce Committee Chairman Jay Rockefeller (D-WV) told CQ:

"It's infuriating," said Rockefeller, who backs Holdren and Lubchenco's hiring. "They're brilliant scientists."

Science advocates said they are frustrated by the blocking of administration officials they argue are needed now more than ever. The holdup could slow timely science and environmental policy work between Congress and the administration, particularly the spending of roughly $21.5 billion dedicated to science in the economic recovery package.

The anonymous senators holding up Holdren, a former Harvard professor, and Lubchenco are not required to unmask themselves until six days of Senate sessions have passed. But nothing prevents reporters from trying to lift the veil...

Late Update: Asked if the anonymous holders in question are mostly Republican senators -- which would be a safe guess, given the nominees' progressive stance on climate change -- Rockefeller told me: "I think so, but I'm not sure." The mystery continues, for now.