HARRISBURG — The criminal trial of former General Assembly dealmaker Michael R. Veon and three of his ex-aides, which begins in earnest today with opening statements in a Harrisburg courtroom, represents a milestone in the attorney general’s three-year public corruption investigation of the Legislature.

The case is also a significant factor in this year’s Pennsylvania gubernatorial campaign and should give voters more details about how some of their state lawmakers have balanced their legal obligations with their political ambitions.

A focus of the trial is expected to be testimony by at least some of the seven people, all former House Democratic staffers, who were arrested with Veon and the three others in July 2008 but who recently pleaded guilty and are cooperating with prosecutors. The proceedings follow a full acquittal won in December by the only defendant so far to go to trial in the investigation, former state Rep. Sean Ramaley, D-Beaver.

The defendants — Veon, 53, once the No. 2 ranking House Democratic leader; campaign guru Brett W. Cott, 37; computer services supervisor Stephen A.H. Keefer, 39; and Veon district office manager Annamarie Perretta-Rosepink, 47 — are accused of conflict of interest, theft and conspiracy in what the attorney general’s office alleges was a multimillion-dollar effort to siphon off public resources for private benefit. The trial is expected to last about a month.

The four and Ramaley are among 25 people charged in the probe that began in early 2007 with news that millions in bonus payments had been quietly handed out to hundreds of employees of the Legislature.

The secret bonuses gave the matter the informal nickname of “Bonusgate,” but the broader investigation involves much more — including how nomination petitions were circulated, the purchase of millions of dollars worth of computer equipment, state employees going onto the campaign trail and allegations that evidence has been concealed or destroyed.

Veon and the others were the first to be arrested, but in recent months three more people with ties to the House Democrats and 10 connected to the House Republican caucus also have been charged. Kevin Harley, spokesman for the attorney general’s office, said as recently as Friday that additional arrests are still expected.

Prosecutors have portrayed Veon as being at the center of the alleged conspiracy among House Democrats. Although they have outlined parts of their case during the preliminary hearing and in extensive grand jury reports, a mountain of e-mail and other evidence has been gathered and could become public during the trial.

Veon’s lawyers have not been shy about charging that Attorney General Tom Corbett has let his dream of being elected governor this year interfere with his judgment as a prosecutor. Corbett is the leading candidate for the nomination in the May 18 Republican primary.

“It’s daunting to fight the power of the state,” said Veon’s lawyer, Joel Sansone. “But the truth is more powerful than the stolen power of people with political ambition.”

Harley said the attorney general’s office has simply followed the evidence turned up by the public corruption unit Corbett promised to establish when he ran in 2004.

“He’s doing his job as attorney general, and that’s what prosecutors and agents do — they investigate,” Harley said. “If there’s probable cause, they file charges. That’s our system, that’s the way the process works, and that’s what he’s doing. He’s fulfilling his oath of office.”

Another criticism has been that relatively few of the alleged decision-makers have been charged. In addition to Ramaley and Veon, Corbett’s office also has charged two former House speakers who still serve in the body — Bill DeWeese of Greene County and John Perzel of Philadelphia — and two former House members, Steve Stetler of York County and Brett Feese of Lycoming County.

“By Mr. Corbett’s own multiple statements, these practices were widespread and long-standing. You might even say historically common,” said Keefer’s lawyer, Bill Fetterhoff. “And therefore yes, it’s not only troubling that employees were charged at all, but it is troubling that a mere handful of the elected members and leaders have been charged.”

Bryan Walk, Cott’s lawyer, said he will argue that Cott’s actions were minor, that he lacked any criminal intent and “that he only did what he was required or asked to do.”