Retiring Sen. Tom Duane is poised to announce he’s backing Assemblyman Guillermo Linares’ primary challenge to Duane’s Democratic colleague, Sen. Adriano Espaillat, a source familiar with his decision confirmed.

This move is not intended as a personal slight agaisnt Espaillat, the source said, but rather is due to the long-standing relationship Duane has had with Linares. (Espaillat and Duane didn’t serve long together in the Senate, as the former was elected in 2010 to fill the seat vacated by now-AG Eric Schneiderman, while Duane has served since 1998. Linares took Espaillat’s old Assembly seat in 2010).

Linares and Duane were members of the same NYC Council class, (elected in 1992), and grew close during their years there as participants in a small, yet vocal, group that consistently voted “no” on the budget deals reached by then-NYC Mayor Rudy Giuliani and Council Speaker Peter Vallone Sr.

In the release announcing his decision, Duane says:

“I’m proud to endorse my friend Guillermo Linares for State Senate. He will be a strong progressive voice for the West Side and Northern Manhattan in the Senate and a fighter for reform as he has been throughout his career.”

“Guillermo and I served together on the City Council, and stood together against the harsh social policies of the Giuliani era. Whether it was protecting the poor, preserving vital services for people with HIV/AIDS, or standing up for the rights of tenants, Guillermo was always there.”

“In the State Assembly, he’s been a champion for so many progressive causes—from the battle for marriage equality to the fight he continues to lead as the prime sponsor of the New York Dream Act. He’s a coalition-builder who knows how to get results—and I know his voice will become an important one in the Democratic conference.”

Roughly 15 percent of the newly drawn 31st Senate District, over which Espaillat and Linares are now battling, is located in the district Duane has represented for the past decade. Another 15 percent or so is in Sen. Bill Perkins’ district. Perkins has so far stayed out the Espaillat-Linares showdown.

Linares announced in May that he intended to seek Espaillat’s seat this fall regardless of the outcome of the senator’s primary challenge to Rep. Charlie Rangel. Espaillat failed in his effort to oust Rangel, and, thanks to a quirk in the political calendar (the Legislature failed to move the state-level primaries to correspond with the court-ordered June 26 House primaries he’s now running for re-election to his current post.

Linares backed Rangel against Espaillat, and the congressman is now returning the favor. Linares also has the support of his former boss, Mayor Bloomberg.