As NY1 first reported last week, former Congressman Michael Grimm will announce on Sunday that he's running his old seat, launching a primary challenge against Rep. Dan Donovan. Grimm's candidacy threatens to upend the careful planning of Staten Island's Republican leaders. Borough Reporter Amanda Farinacci filed this report:

Michael Grimm's decision to run for his old seat in Congress is upending Staten Island's Republican Party.

It's not just that he's challenging the fellow Republican who replaced him, Dan Donovan. He's also threatening the plans of the borough's GOP machine - insiders who prefer to decide among theselves who gets what office, and when. 

"When you start having and devising these plans, this one's gonna do this three years from now, this one's gonna do that and if it's done just to, as a personal, this is my friend, I'm gonna take care of my friend. Well, then, sooner or later that's gonna unravel," said former Congressman Vito Fossella.

Fossella held the seat for a decade. No longer an active member of the island's Republican club he's backing Donovan, calling him an effective representative.

But he points to Trump-era politics of "anything goes" as reason Donovan should take a challenge seriously. Grimm plans to run to the right of Donovan, portraying his replacement in Congress as unfaithful to core Republican principles.

"I just would not take anything for granted in this political climate," Fossella said.

Grimm resigned in 2015 after pleading guilty to tax fraud and serving seven months in a federal lockup. 

He says he was the victim of a politically-motivated witch hunt.

Many voters say they are open to his candidacy.

"Grimm has a certain charisma that Republican voters like. He won the seat when he was under indictment by a pretty big margin. He's formidable," said Richard Flanagan, a professor at the College of Staten Island. 

Grimm is the protege of former Borough President and Congressman Guy Molinari, who has long supported him. Absent that backing, it is unlikely Grimm will win an endorsement from any elected official on the island.

As rumors of Grimm's candidacy spread, the borough's Republican party sent out a press release, with the headline "We Stand with Dan." Every elected Republican in the district signed it, including City Councilman Joe Borelli, long seen as the machine's choice to succeed Donovan one day.

But do the endorsements matter?  

"I don't think they carry much weight. And they can be turned around and used to a politician's advantage: he's an outsider," Flanagan said.

It all points to a heated intra-party battle - pitting a maverick politician against the machine, with the political order riding on the line.