A Metro train in Washington, D.C. (Jabin Botsford/The Washington Post)

IN THE defining act of his governorship to date, Maryland Gov. Larry Hogan on Thursday set aside partisan and regional politics by giving his approval to the Purple Line transit project, an east-west light rail that would connect Montgomery and Prince George’s counties. At the same time, he shifted so much of the project’s cost to the two counties, and, most likely, to the prospective private-sector builders and operators, that its ultimate success remains in doubt.

The governor, who was a Purple Line skeptic during his campaign last year, deserves credit for recognizing that, on the merits, the project should go forward.

It took political guts for Mr. Hogan, a Republican who collected most of his votes from rural and exurban voters, to embrace an urban transit project that will overwhelmingly benefit Democratic suburbanites who didn’t vote for him. (Simultaneously, Mr. Hogan made another good call by killing a major transit project in Baltimore, the Red Line, which never made transportation or financial sense.)

He did so in large part because of the Purple Line project’s obvious and multiple economic benefits, which will generate jobs and revenue and also revive aging, close-in suburban neighborhoods in the state’s most economically dynamic region. Mr. Hogan, who repeatedly pledged that he would make jobs and economic renewal his No. 1 priority, proved by his decision that he meant it — and that he is willing to risk alienating his GOP base in the process.

Yet all that is true only if the Purple Line is actually built. And Mr. Hogan, who declined in a news conference to provide critical details about the project’s financing, remains a long way from ensuring that happens.

Officials said more than $200 million has been lopped from the $2.5 billion project. Mr. Hogan said the state also had somehow offloaded some $530 million of what had been its $700 million share.

Some of that cost, perhaps nearly $100 million, would be shifted to Montgomery and Prince George’s. That may be doable for Montgomery; for Prince George’s, where finances are shakier, it would be a tall order.

As for the balance, Mr. Hogan made it reasonably clear the state is hoping the project’s eventual private-sector builder would cough up a considerable chunk of cash, in return for running the rail line and making profits from it well into the future. It remains to be seen whether those numbers add up and whether project bidders will still regard the Purple Line as a good bet given the state’s paltry contribution.

It also remains to be seen whether the project can attract the $900 million in federal transit funds that the governor said was necessary. Congress has earmarked $200 million of that sum so far; there is no guarantee regarding the balance.

That’s a lot of unknowns. Still, Mr. Hogan has set the bar for himself by acknowledging that the project makes sense and that its benefits — not least, thousands of jobs simply to construct the line over the next six years — are undeniable. It is now incumbent on him — in addition to federal, county and private-sector partners — to get the job done.