For years, whenever State Sen. Vincent J. Fumo strode before the TV lights for a big announcement, Howard J. Cain was apt to be standing against the back wall - a hefty, bald man in a blue suit and suspenders with his arms folded.

Cain, genial and quick-witted, with a voice that sounds as if he's perpetually whining, was a member of a small circle of friends, aides and confidants who depended on Fumo for much of their livelihood and gave him fierce loyalty in return.

In 1999, during a brief tiff when Fumo and Cain found themselves on opposites sides of a Philadelphia mayoral primary, Fumo told the Philadelphia Daily News, "Howard is family."

But now Cain is under criminal investigation and has begun cooperating with the U.S. Justice Department in its 139-count corruption indictment against Fumo, according to sources quoted in an Inquirer story yesterday.

Cain, 59, began cooperating with the investigators after he came under scrutiny for possible tax violations, the sources said. If he testifies against Fumo, Cain would be the first of the inner circle to give evidence against their mentor.

In recent months, Cain has told associates that he and Fumo had quarreled. The issue was whether Fumo, wounded by the corruption probe, could nevertheless win reelection this fall. Cain thought not, a view he believed irked the senator.

Cain yesterday said he had no comment on any contact with federal authorities or his relationship with Fumo.

"I've talked to my attorney," he said. "He advises me I should not say anything. At this time, I am taking his advice."

Cain is 59; Fumo, 64. For two decades, the pair have worked together on everything from getting Fumo re-elected to his Senate seat to exercising Fumo's power in the Senate and helping Fumo allies win elections.

As the powerful Democratic leader of the Senate Appropriations Committee, Fumo was always interested in helping other Democrats win Senate or House seats. Cain was often dispatched to western or northern Pennsylvania to lend a touch of Philadelphia hardball to a rural or small-town election contest.

The government alleges that, at times, Fumo used state money to compensate Cain for political work. From 2000 to 2005, the indictment says, the Senate paid Cain $463,500.

Philadelphia media consultant Larry Ceisler said Cain is "as good an election-day field operative as I have ever worked with."

"He could really put an election-day operation together in terms of placing people, getting out the vote, handing out the money," said Ceisler, who worked alongside Cain on Rick Mariano's first race for City Council in 1991.

Ken Smukler, a campaign strategist who has also worked with Cain, called him "one of the smartest field tacticians in Philadelphia."

Said Smukler: "He could not only take you through every ward and division, but give you the vote history by those wards and divisions going back 20 years. . . . If I needed a guy to lead a field operation in Philadelphia, he is the guy I would pick."

As a wide-ranging consultant, Cain had clients separate from Fumo, including labor unions, boards and authorities. But his interests rarely collided with Fumo's.

One time where they did butt heads was in the 1999 Democratic primary for mayor. Cain backed John White Jr. while Fumo was raising money and spending political capital for Marty Weinberg.

That fall, after both White and Weinberg had lost to the eventual mayor, John F. Street, Cain sided not with the Democrat but with Republican Sam Katz. Some saw that as another split with Fumo.

But others saw it as a back-channel attempt by Fumo, who was not close to Street, to aid Katz. Fumo denied it.

Cain over the years was also involved in projects to build up the Port of Philadelphia.

He was a member of a team that succeeded in having Philadelphia declared a strategic port by the Department of Defense and a home port for an advanced type of Navy vessel - the large, medium-speed, roll-on/roll-off ship (LMSR).

"We were fighting a really uphill battle against Boston and Baltimore and New York," said Susan Howland, president of the Howland Group, the project manager for the Delaware River Maritime Enterprise Council.

"I personally like Howard," she said. "He's humorous, he works really hard. He really personally believed in what we were trying to get done."

Cain also had his own interests on the riverfront.

Early in the Fumo investigation, federal agents subpoenaed the records of the Ship Recycling Research Institute, which was headed by Cain and operated out of the Philadelphia Naval Business Center at the Navy Yard.

Cain has said the institute was founded in 2002 to promote ship recycling, the process of scrapping and reusing obsolete maritime vessels.

Cain had described the group as a cross between a think tank and a matchmaker, trying to pair the Navy with coastal communities that want to sink decommissioned ships for artificial reefs.

The institute received a $500,000 loan from a nonprofit, the Delaware Valley Regional Economic Development Fund. That fund gives out grants and loans, using money that Fumo arranged for it to obtain from Peco Energy.

As president of the group, Cain was paid $60,000 annually. It is unclear whether Cain is still affiliated with the institute.

In recent years, a Democratic elected official said yesterday, Cain appeared to distance himself little by little from electoral politics.

The official, who asked not to be named, said Cain seemed to sense that Fumo was headed for legal trouble with some of his political activities.

"There was something about being in politics that was either scaring him or making him depressively negative about it," the official said. "He expressed dissatisfaction with being a political operative."