According to witnesses, the planning occurred in Washington's Senate offices in Philadelphia, Rosyln and Harrisburg.
Washington, a Democrat who has represented Northeast Philadelphia and south Montgomery County for 20 years, has denied any wrongdoing.
After a multi-county grand jury indicted Washington, the Attorney General decided to file charges in Montgomery County because the Roslyn office "was a hiding place for some of defendant's political activities," DiGiacomo wrote.
At a preliminary hearing in March, former Washington staffer Jamila Hall testified that the party planning initially was done in the Philadelphia office.
"It would usually be in the back office, but the years that I gained more responsibility, I would go to the Roslyn office," Hall said.
"Who instructed you to go to the Roslyn office?" DiGiacomo asked.
"The Senator," Hall said.
"And why did she instruct you to go to the Roslyn office?"
"Because Sean wasn't there," Hall said, referring to Sean McCray, the former chief of staff who blew the whistle on Washington's alleged misconduct.
McCray told prosecutors that when he questioned the senator about the political work, she berated him and later fired him. He has since filed a lawsuit alleging defamation and unlawful termination.
Washington, through her attorney, has called McCray "a disgruntled former employee."
Although it was not brought up in the change of venue motions, McCray told The Inquirer last week that the only reason he approached Montgomery County prosecutors instead of Philadelphia prosecutors was that the suburban county was smaller and easier to deal with.
Washington's attorneys cited a letter from Montgomery County District Attorney Risa Vetri Ferman as support for their change of venue request.
"Most of the political and campaign activities alleged to have been done on government time occurred in Harrisburg or Philadelphia," Ferman wrote. "As such, I believe your office is in the best position to continue to investigate these allegations against Sen. Washington."
DiGiacomo said Ferman was right to pass the investigation on to the Attorney General, but that doesn't mean Montco isn't the right venue for trial.
"The defendant can cite no reason why the residents in Montgomery County would not give her a fair and impartial trial," she wrote.
If convicted on the two felony counts of theft of services and conflict of interest, Washington could face up to 12 years in prison. She will leave office at the end of this year, after losing her primary re-election bid in May.