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Senate Democratic leaders didn’t respond to a request for comment on the GOP’s legal move on Wednesday.

In a curious twist, Republicans have hired Miguel Estrada to be their lawyer. Mr. Estrada was once a nominee for a top judicial post, but Senate Democrats filibustered to block his nomination, making him the first casualty of the partisan judicial nomination wars under Mr. Bush.

No Democrats joined the GOP in the lawsuit, and five Republican senators also didn’t sign on.

Two of them, Sens. Scott P. Brown of Massachusetts and Dean Heller of Nevada, are locked in tight re-election battles back home and have somewhat distanced themselves from their colleagues. Retiring Sen. Olympia J. Snowe, a Maine Republican who sometimes sides with Democrats, didn’t join, nor did Sen. Lisa Murkowski, an Alaska Republican who tussled with her party in the 2010 elections.

Sen. Mark Kirk, Illinois Republican, is still recovering from a stroke and hasn’t been in the Senate chamber for more than a year.