But it’s not, and now the Regional Transportation Investment District board finds itself facing a choice between two imperfect alternatives. Both include asking voters for partial funding for the cross-base highway, but only one appears to risk rallying a large and potent force in opposition.

The original plan was to include the cross-base highway — minus $100 million that would have to be found later to complete the project — in the regional roads and transit plan that will appear on Puget Sound voters’ ballots this November.

But after a coalition of environmental groups threatened to oppose the entire package if the cross-base highway were left in, RTID leaders came up with another idea: Drop the highway for now but go ahead with $407 million in road and freeway improvements at both ends of the proposed route.

Choosing between the two — which the RTID executive committee will do Thursday — comes down to whether you believe that the state’s environmental lobby will follow through on its threat and whether you believe that opposition could be enough to kill the entire package.

We are convinced that the threat is real and that too much else is at stake to gamble it all on a game of hardball.

Cross-base is but one piece of a $20 billion regional plan that will add highway capacity and build more mass transit across Pierce, Snohomish and King counties. The regional perspective compels transportation leaders to not let controversy over this one project obstruct the rest.

It’s regrettable that the environmental community has resorted to blackmail to get its way on a package that already has so much to offer their interests. If the proposed $8.7 billion for regional road and bridge work goes down, so too does an even bigger — and potentially more important — $10.8 billion investment in Phase II of Sound Transit’s system.

But given the potential of jeopardizing this region’s best shot at making meaningful progress on shoring up its beleaguered transportation system, the wise choice is to fall back and live to fight the cross-base battle another day.

The revised plan still gets the cross-base project 60 percent of the way there. Improvements to I-5 interchanges at one end and 176th Street and Canyon Road at the other would provide the foundations for completing the project, as well as deliver more immediate relief than the original RTID plan. (It assumed cross-base highway construction wouldn’t begin until 2017). Boeing officials, who operate a plant in the Fredrickson industrial area, have told RTID leaders that they would have much to gain from speeding up the work in that area.

In the end, the cross-base question is a political one, and politics is the art of the possible, not the perfect. RTID leaders face a far better chance at the ballot this November if they can negotiate a truce now rather than head into the campaign with well-heeled opposition. As much as it might rankle, pragmatism is the order of the day.