Frustrated by efforts to launch a Congressional investigation into how a controversial earmark was attached to the 2005 highway bill, Senate Democratic leaders have ramped up their attacks on Sen. Tom Coburn (R-Okla.), the proposal’s primary sponsor, accusing him of attempting to politicize the issue.

Coburn and a bipartisan group of Senators over the last several days have attempted to attach an amendment to a highway technical corrections bill that would create a bicameral committee to investigate how and why Rep. Don Young (R-Alaska) — who was Transportation and Infrastructure chairman at the time of the 2005 bill — inserted the $10 million earmark, which would benefit a wealthy Florida land developer and a campaign contributor to Young.

Starting at 3:30 p.m., the Senate is expected to vote on an amendment by Sen. Barbara Boxer (D-Calif.), which is supported by Democratic leaders, to launch a Justice Department probe into the earmark. The Coburn amendment will come to the floor after that, followed by a vote on the underlying legislation.

After nearly 24 hours of quasi-negotiations with Coburn, Boxer accused the Republican of attempting to kill the technical corrections bill, which she cast as a “mini economic stimulus package.”

Ahead of the vote, the Senate Democratic Policy Committee circulated a memo to Members’ offices questioning the constitutionality of Coburn’s bill because it would have the Senate investigate a House Member.

The memo cast the Coburn measure as a political “poison pill” and said Senate Majority Leader Harry Reid (D-Nev.) “strongly supports the Boxer amendment and strongly opposes the Coburn amendment.”