Tom Giblin will not run again in LD-27. In a significant change of course in a battleground primary district, Democrats are poised instead to run Alixon Collazos.

According to a source, Giblin, a veteran assemblyman from Essex County and longtime party leader, will not pursue reelection to the legislature, after all.

Redistricted out of the 34th, he was geared up to go, preparing to run on the 27th District ticket with state Senator Dick Codey and Assemblyman John McKeon. The three men appeared ready to face a Democratic Primary slate headed by incumbent state Senator Nia Gill, who is running for the 27th District senate seat occupied by Codey.

Gill is running with Eve Robinson and Frank Kasper.

Essex remains on target for a contest pitting Codey – who will have the backing of the Essex County Democratic Committee – against Gill.

But now Giblin won’t run, sources say, supplanted by Collazos who, like Giblin, is also from Montclair.

A labor leader and former Democratic State Party chairman with deep Essex roots, Giblin has served as an assemblyman since 2006. He also served as Essex County Democratic Party chairman, from 1993 to 2003, and lost a donnybrook of a 2002 contest to Joe DiVincenzo for county executive.

An apparent casualty of last year’s redistricting map, state Senator Gill and Essex County Democratic Committee Chairman Jones never really got along, going back at least to 20o3.

That was the year a fight erupted in Essex County as Assemblyman Jones – the future county Democratic chairman – opposed Gill in the Party Primary as the newly minted senator attempted to secure a full, four-year term.

Jones ran on the line with Essex County Freeholder Sheila Oliver of East Orange, and Peter C. Eagler of Clifton. Financed in part by state Senator Ray Lesniak (D-20), Gill beat Jones by 1,000 votes, but it was a split ticket outcome, as both Oliver and Eagler (who replaced Willis Edwards) also won their assembly seats.

In 2005, Assemblyman Giblin supplanted Eagler as Essex dominated the 34th District.

Now, heading into a tough primary season, Collazos will join veterans Codey and McKeon in their showdown with Gill’s ticket, sources say.