The route to building or rejecting a southwesterly Green Line light-rail extension has taken so many turns that future historians of public policy may have trouble determining which moves mattered to the ultimate result. But chances are good that a decision that came to light last week will loom large in hindsight.

The decision: The state’s final $138 million share of the $1.74 billion project will be appropriated — or denied — by a vote of the politically divided 2016 Legislature. An end run that would have averted the need for legislative approval will not be attempted, Metropolitan Council chair Adam Duininck said.

That’s a decision that has supporters of the Southwest LRT project more than a little nervous. Transit in general and light rail in particular have been much resisted by the Legislature’s Republicans, who control the House and whose votes are needed for bonding bills in the DFL-controlled Senate as well.

The Star Tribune Editorial Board strongly supports the Green Line’s extension, and we share the jitters about the project’s fate in legislative hands. Still, we think Duininck and the council have chosen appropriately. A decision to spend $138 million in state taxpayer funds — plus interest, should the state issue bonds for that amount — for a controversial project ought to be made by elected representatives, not by the Met Council’s gubernatorially appointed 17 members. Southwest LRT ought to win at least bipartisan acquiescence, if not approval, before proceeding.

The funding mechanism that’s been rejected involves certificates of participation, which are purchased by investors under a promise that future government revenue will repay them with interest. Unlike general obligation bonds, certificates don’t carry a constitutionally mandated repayment mechanism to prevent default and don’t require super­majority approval by the Legislature.

Duininck said earlier this year that the Met Council already has legal authority to use certificates of participation for Southwest LRT. But he called it a “last resort,” given the council’s dependence on the Legislature to fulfill its larger mission — operating Metro Transit, treating wastewater, developing and operating regional parks and trails, providing affordable housing and planning for the region’s future.

“The Legislature is an important partner” for this project, Duininck said Wednesday about the decision to drop the last-ditch option. “It’s important to engage the Legislature in this debate.”

Duininck and the council may have taken a lesson from the most recent use of certificates of participation. They are financing the new Minnesota Senate office building across from the Capitol — a project that became a cause célèbre in 2014 legislative races, to the DFL’s disadvantage. If certificates were to be employed for Southwest LRT without a legislative vote, Republicans in the Legislature would be sure to cry foul and carry their objection into the 2016 election, in which both legislative chambers are on the ballot.

It’s fair to assume that Republicans would not confine their criticism to transit funding. The structure, authority and very existence of the Met Council likely also would be on the line in the 2016 election if Southwest’s go-ahead came without legislative approval.

Duininck’s decision deprives the GOP of a potent talking point in the 2016 campaign. But it assures a high-stakes battle at the 2016 Legislature over rail transit’s future in the Twin Cities. Southwest’s supporters — particularly in the business community — should be on notice: This project is going to need all the advocacy and lawmaking agility they can muster next session.