Move Scott Stringer from would-have-been prospective primary challenger for Sen. Kirsten Gillibrand to strong supporter. On Monday, the Manhattan borough president is set to announce his endorsement of the woman he once hoped to unseat—and with it, his personal certification of the junior senator’s progressive credentials.

“Obviously, she has successfully gone through her own evolution on a lot of issues and has said she’s reaching out and learning every day—and I think that’s sincere and real,” Stringer said, explaining his decision. “I’m actually excited to go out and campaign for her.”

Stringer cited collaboration on several issues, including the environment and a connection between upstate agriculture and his own food shed proposal, as those which convinced him to back the woman he was once hoping to unseat.

Gillibrand has one announced Democratic primary challenger—activist Jonathan Tasini—and Suffolk County Legislature majority leader Jon Cooper is still weighing whether to run.

But Stringer made clear that he is already looking past that.

“She’s going to be the Democratic nominee, and my job is to make sure that she’s successful and continues to grow as a senator,” Stringer said.

In March, Stringer announced that he was forming an exploratory committee to take on the appointed junior senator, whom he said should receive a challenge.

“I think the reason we have elections is to exchange ideas on the important issues of our time,” Stringer told the New York Times then. “Voters have an expectation for a Senate seat held by Kennedy, Moynihan, that there will be a discussion of these issues through the electoral process.”

Stringer ended his flirtation with running in mid-May, in the wake of President Barack Obama and his chief of staff Rahm Emanuel urging Long Island Rep. Steve Israel to abandon his own candidacy.

Stringer’s decision to support Gillibrand comes two weeks after she received the endorsement of Stringer ally Rep. Jerry Nadler, a fellow Upper West Sider and high-profile progressive. Nadler was the first to back Gillibrand of those who had been angling in January to get appointed by Gov. David Paterson to what was Hillary Clinton’s Senate seat. Stringer is the first of the people who had been actively taking steps to challenge her in next year’s primary to make his endorsement.

Stringer said that he has seen evidence over the months since he opted out of the Senate race that has convinced him that Gillibrand is committed to the right issues, ready to work with him on issues that concern him and in line with his progressive ideology.

“She has demonstrated broad-based support in the progressive communities,” the borough president said.

As to whether that meant he now considered Gillibrand a progressive, Stringer said: “Yes.”