GOP mayoral hopeful Paul Massey on Wednesday proposed extending the G train into Manhattan during the 15 months when the L train is shut down for superstorm Sandy-related repairs.

Massey’s plan, which he called the “G Loop,” would extend the train from its current terminus in Long Island City, Queens, into Manhattan, through midtown to downtown and back to Brooklyn.

At various points the G would run on the tracks as the F and M. It’s unclear the impact the additional line would have on those existing ones. The plan would require the construction of a new tunnel between tracks north of existing tracks in Queens.

“This is a lifeline for L train riders, immediately increasing connectivity to the rest of the city,” Massey, a real estate executive, said at a news conference in Williamsburg.

The plan would cost “millions, not billions,” Massey said.

If elected, Massey would have no direct power to implement the plan, because the MTA is controlled by the state and governor. Massey said he’d be able to work with Gov. Andrew M. Cuomo better than Mayor Bill de Blasio. “Our mayor hates the governor,” Massey said.

MTA spokesman Kevin Ortiz dismissed the idea, which has been proposed in the past. “It is not feasible,” he said, because of other trains already using the tracks, there’s not enough space for necessary switches and other roadblocks. Aspects of the plan, he said, “would be highly disruptive if they were feasible at all.”

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De Blasio campaign spokesman Dan Levitan declined to comment.

The Metropolitan Transportation Authority is planning to suspending L train service — which connects Brooklyn neighborhoods like Williamsburg to Manhattan — beginning in April 2019 for 15 months to repair a tunnel under the East River devastated by Sandy in 2012.

Massey rode to and from his transit announcement aboard the L, handing out campaign literature blaming the “failed leadership” of de Blasio for the system’s latest woes.

After a brief chat with Massey on the subway platform and a skim of Massey’s literature, Yadira Nieves, 42, of Williamsburg, said she’d vote for Massey in November.

“I like de Blasio,” she said, “but I need a change.”

But Alexander Dimitriyadi, 28, a software engineer who lives in the Financial District, said although he voted for de Blasio’s 2013 Republican opponent, Joe Lhota, he wouldn’t pick Massey.

“He’s no one, really,” Dimitriyadi said of Massey, who a Quinnipiac poll said would lose to de Blasio 21 to 63 percent, despite expenditures of millions of dollars.

“He’s spending a lot of money,” Dimitriyadi said aboard Massey’s train. “Not seeing a lot of results.”