Ten-term Rep. Donald Payne (D-N.J.) has never won with less than three-quarters of the vote in his successful House elections, and in recent cycles the Newark-based Congressman has won with vote margins of upwards of 90 points.

So it might come as a surprise to learn that some New Jersey political insiders are now calling Payne the most vulnerable member of the Garden State Congressional delegation.

Though no one else has filed for the race, these insiders believe that if a Democratic primary challenger emerges with the support of Cory Booker, the high-profile mayor of Newark whose unsuccessful bid for City Hall in 2002 was the subject of the Academy-Award-nominated documentary “Street Fight,” Payne could be in trouble.

Earlier this summer, PoliticsNJ.com, a Web site that covers Garden State politics, noted that “the list of Democrats growing tired of the Payne political organization is rapidly increasing” and that with the rise of Booker, Payne has “found himself on the wrong side of an Essex County Democratic political battle.”

Since winning his 2006 election to replace Sharpe James — the man Booker lost to in 2002 and who has since been indicted by a federal grand jury on fraud and conspiracy charges — Booker has pushed a reform agenda in Newark.

Meanwhile one New Jersey political operative noted that Payne is still viewed as part of James’ organization.

Since taking office, Booker increasingly has been throwing his support to candidates in state and local races who are challenging Payne-backed candidates.

In June, incumbent state Assemblyman Craig Stanley, who is Payne’s nephew, lost in the Democratic primary in a close race with Cleopatra Tucker, who was Booker’s pick for the seat.

Meanwhile in the general election for a state Senate seat in New Jersey’s 29th district, Payne’s brother William — a state Assemblyman who after being passed over in the Democratic primary is running as an Independent — will face off against Teresa Ruiz, the Democratic nominee.

During the campaign, both Ruiz and William Payne are taking a leave of absence from their jobs as as deputy chiefs of staff for Essex County Executive Joseph DiVincenzo (D). But it is Ruiz who earned DiVincenzo’s backing and the endorsement of Booker.

Donald Payne is backing his brother.

Though political alliances can change quickly in New Jersey politics, Booker’s current partnership with DiVincenzo and North Ward Democratic leader Stephen Adubato — a major player in state politics — is being viewed as a powerful force in Payne’s district, of which Newark makes up about 60 percent. And even though there are several other candidates in the state Senate race, some are looking to the fall contest between William Payne and Ruiz as a barometer of what could happen during next June’s primary for the Congressional seat.

Meanwhile, adding fuel to the fires of speculation about the future of Donald Payne’s House seat is the fact that earlier this spring Booker publicly declined to commit to supporting the Congressman for re-election in 2008.

One name that has been floated as a possible primary challenger to Payne is Elnardo Webster, a friend and former business partner of Booker’s who had a brief career in the National Football League.

Payne spokeswoman Kerry McKenney said Monday that her boss wasn’t totally surprised by talk of a possible primary challenger in 2008.

“Mayor Cory Booker is putting up his own candidates in races and he’s declined to confirm his support for Congressman Payne,” McKenney said. “But [Payne] feels confident he can run on his record. He’s prepared for a challenge.”

Indeed, any Booker-backed candidate would start with a large funding disadvantage, as Payne had more than $770,000 in the bank through the end of June.

Meanwhile, even those involved in the race cautioned against reading too much into the future implications of the Ruiz/Payne state Senate race.

Philip Alagia, who works for DiVincenzo and is managing Ruiz’s campaign, said the state Senate race shouldn’t be viewed as a Booker vs. Payne election. Booker “is one of many pieces of the puzzle supporting [Ruiz]. And a lot of people who are supporting this team are going to be supporting Donald Payne next year,” Alagia said. “There’s not clearly a line in the sand and it’s not us against them.”

McKenney said Donald Payne is well aware that William Payne is going to face a “very challenging” race to win as a third-party candidate against Ruiz. But, she said, Donald Payne “doesn’t expect [his brother’s race] to directly impact his own race.”

Other New Jersey political insiders also are questioning just how much political clout Booker has in Newark right now.

In the Democratic primary in another state Senate district this year, Adubato and Booker fielded a candidate who lost to state Sen. Ronald Rice, who had the backing of the Payne family.

“If [Booker] couldn’t bump off Ron Rice … I’m not sure that he could bump off Donald Payne,” another New Jersey political insider said.

Meanwhile, the same source noted that Booker ran on a platform that promised to lower the violent-crime rate in Newark. And after last month’s high-profile murders of three teenagers at a Newark schoolyard, that concern remains high, and Booker may not be able to afford the luxury of getting involved in the Congressional primary.

“If the crime situation does not get reversed by next June, you’re looking at big problems for Cory Booker and he knows it,” the source said.

But Payne’s own political activities can be a mixed blessing in advance of his re-election, another knowledgeable political observer said.

“I don’t necessarily think Payne is vulnerable,” the source said. “The only thing that makes him vulnerable is that he has had a tendency not to maintain a full-scale political operation in the district. This Senate battle will in some senses compensate for that because those forces will be put to work and we’ll see how it goes for his brother.”