Senate Democrats on Tuesday will hold a vote to force President Barack Obama’s hand on the Keystone XL, the controversial pipeline that would carry crude oil from the Canadian tar sands.

This move has baffled environmental activists, who have spent six years campaigning against the pipeline and sunk $85 million into defending Senate control. It’s also baffled Mother Jones’ Kevin Drum, who called it “crazy,” and the American Prospect’s Paul Waldman, who described it as a “politically idiotic” move. (My choice of phrase was “insanely dumb.”)

Senate Democrats are holding this vote now in part to boost Senator Mary Landrieu’s prospects in her December runoff, which she’s likely to lose. They are also holding it because they know, with certainty, that Republicans will make this their first priority when they take over the Senate in January. As my colleague Brian Beutler points out, soon-Minority Leader Harry Reid doesn’t want to be the same obstructive party Republicans have been throughout Obama’s presidency. This could be the opening move in the strategy.

Democrats now intend to take this vote away from them. By Politico’s latest count, 58 Senators support the bill, following Senator Tom Carper’s decision to change his vote. The House of Representatives is rushing its own version of the bill.

No one knows for sure what Obama would do, but the signs point to his vetoing the bill. “The administration, as you know, has taken a dim view of these kind of legislative proposals in the past,” White House Press Secretary Josh Earnest said Wednesday. “I think it’s fair to say that our dim view of these kinds of proposals has not changed.” Of course, Obama could either sign the legislation or leave it unsigned, which has the same effect of making it law.