In many Philadelphia political circles, it's considered a given: Chaka Fattah is a goner.

Sooner or later (and probably sooner), a long-running federal investigation will likely force the 10-term Philadelphia Democrat out of Congress. Or so went the insider thinking among politicos and lobbyists this month at the annual Pennsylvania Society parties. They even talked of who might succeed him.

Although the probe of Fattah's use of federal earmarks has gained steam in recent months with guilty pleas from two former campaign aides, legal experts say building a case against any sitting congressman may be more difficult than it first appears.

Former prosecutors cite a litany of potential impediments to the investigation, including provisions shielding congressional acts from prosecution and the sheer complexity of the guilty pleas.

"The overwhelming presumption among all the gossipers . . . is that Fattah at some point will not be a congressman," said Neil Oxman, a Philadelphia political consultant. But he called the talk premature.

"Right now, the only people who have been accused of any wrongdoing," Oxman said, "have not been him."

Even as he contends with federal subpoenas and mounting six-figure legal fees, Fattah, 58, who begins his 11th term in January, vows he isn't going anywhere. He has not been charged with anything.

"Unless there's going to be a coup, voters in my district have decided, somewhere around 90 percent of them, that I'm going to represent them," he said, referring to his 88 percent share of the vote in November.

Outside a fund-raising event last week, he said, "I'm not as embattled politically as it may appear."

'Elected Official A'

The smoke around Fattah has thickened.

Prosecutors have painted Fattah, who sits on the House Appropriations Committee, as the orchestrator of a scheme to use federal grant money to pay off a $600,000 debt from his 2007 mayoral run.

Two former aides, Gregory Naylor and Thomas Lindenfeld, admitted their roles and pleaded guilty this year.

Their plea documents say Fattah, identified only as "Elected Official A," routed taxpayer money to nonprofits run by other former staffers, then used that money to pay his campaign debt.

The scheme described in the documents was complicated.

"There are a lot of joints in that machine for a good defense attorney to kind of stick a wrench in there and bring the whole thing down," said Rob Walker, a defense lawyer and former prosecutor for the Justice Department's public integrity unit.

Walker called the plea documents "unusually detailed. That may well be part of an effort to demonstrate to the congressman just how much evidence they [prosecutors] have."

Fattah has said he planned to serve for another 10 years, reiterating that vow at a groundbreaking for public housing last week, at a fund-raiser at the Tavern 17 restaurant in Center City, and in two interviews.

"I have never been involved in any illegal conduct or misappropriation of funds," he said Thursday.

He called this last session of Congress the most productive of his career. In news releases last week, he touted his work to increase funding for two of his top priorities, youth mentoring and neuroscience research.

A former Fattah aide said the congressman was focused on constituents, not the chattering classes. "He's an eternal optimist," said the former aide, who requested anonymity. "He believes fundamentally that people can see his good work."

A costly legal battle

The investigation has taken a financial toll.

Since May 2013, Fattah has paid $114,500 in legal fees from his campaign fund, public reports show. The largest share, $50,000, has gone to his personal lawyer, Luther Weaver.

The campaign fund also paid four other law firms for related matters, including a $25,000 retainer to the Dilworth Paxson firm.

Fattah said the first subpoenas arrived as early as 2008, and he has questioned prosecutors' conduct in the long-running probe.

Although Naylor and Lindenfeld's guilty pleas breached Fattah's inner circle for the first time, there is a difference between targeting aides and members of Congress, defense lawyers and former prosecutors said.

The constitution's "speech and debate clause" shields legislative work from interference by executive branch offices such as the FBI and Justice Department.

Recent court decisions have expanded those protections, much to prosecutors' frustration.

"The framers thought it was important to protect these people against intimidation of the executive branch," said Stanley Brand, a Washington defense lawyer and former general counsel to the House of Representatives.

That means the earmarks prosecutors have cited in the cases against Fattah's aides likely could not be used against the congressman.

Said Brand: "That's going to be a major problem."

Fattah has asserted "speech and debate" protections in his fight against a subpoena of his congressional office, according to sources close to the investigation.

"We're doing everything that lawfully is permitted in terms of our cooperation," Fattah said.

In the 2013 conviction of former U.S. Rep. Rick Renzi (R., Ariz.), prosecutors had to focus on his actions away from Congress, such as extortion and money-laundering, said Andrew Levchuk, a former Justice Department lawyer who helped make the case.

The Supreme Court has ruled that the "speech and debate" clause does not give lawmakers blanket protection for actions outside their official duties.

So prosecutors targeted Renzi's shakedown of developers and mining companies seeking his legislative help.

If prosecutors try to prove Fattah broke laws in the loan scheme described in Lindenfeld and Naylor's guilty pleas, they would have to show that he knew about the plan and that it was not just initiated by aides, Levchuk said.

Hard evidence helps: In the Renzi case, one of the men the congressman leaned on documented the conversation by mailing himself a letter postmarked with the date, Levchuk said.

A high bar

Prosecutors must also weigh the potential political fallout of indicting a sitting elected official.

The damage of losing such high-profile cases was illustrated by the botched prosecution of U.S. Sen. Ted Stevens (R., Alaska), whose 2008 conviction was thrown out - "a black eye on the Department of Justice from which they are still recovering," Levchuk said.

Even criminal charges do not necessarily end congressional careers. U.S. Rep. Michael Grimm (R., N.Y.) was indicted in April on 20 counts of fraud, perjury, and obstruction. He won another two-year term in November.

Officials in Washington have kept close watch on the Fattah probe. Eric L. Gibson, a former Philadelphia prosecutor who now works as a lawyer in the Justice Department's public integrity section, has worked alongside locally based federal prosecutor Paul L. Gray throughout the investigation.

"The government has the wagon train surrounded. They're riding around, picking off people, and sooner or later they're going to be taking aim at the congressman," said L. George Parry, a Philadelphia defense lawyer and former federal prosecutor. But, he added, "If you go to kill the king and you miss, you've got a major mess on your hands."

Though some see a case that is heating up, others question an investigation that has gone on this long without charges against the man at its center.

"It's seven years - you can't bring a case?" Brand said. "It seems unfair."

Fattah pointed out that he keeps getting reelected despite the investigation.

"Me and you," Fattah said, "could be having the same discussion on the eve of my next term."

@JonathanTamari