Washington, 68, a Democratic legislator for more than 20 years who represents parts of Philadelphia and Montgomery County, could face up to 12 years in jail.
The state Attorney General's Office says Washington spent as much as $100,000 in taxpayer funds to plan and promote the fund-raisers from 2007 to 2013. The parties took place around Washington's birthday, in late July.
Washington has vowed to fight the charges, which her lawyer called "thin and specious."
In the courtroom Wednesday, Hall said she never received any ethics training. But based on the senator's instructions to conduct campaign work in a back office, and to hide it from certain employees, Hall said she knew "from common sense" that it wasn't allowed.
Washington sat stoically during most of the hearing. But the senator shook her head as Hall described a toxic work environment filled with "verbal abuse by the senator." Hall said she saw staffers cry, quit, and be fired.
"To go against anything the senator put forth wasn't an option," said Hall. "If you complained, you were fired."
Hall received immunity in exchange for her testimony.
The hearing turned heated when defense attorney Henry E. Hockeimer questioned whether the senator had explicitly instructed Hall to do campaign work on Senate time and using Senate equipment.
Hockeimer suggested that those instructions may have come from Washington's former chief of staff, Denise Savage, who is also cooperating in the case.
Hall insisted that they came from Washington. She recalled the senator telling her to work on the invitations for the party at the less trafficked district office in Roslyn and to have two paid interns help her.
"Go up to the other office, it's quieter. Take the girls and get it done," Washington told her, Hall testified.
On other occasions, Hall said, the senator instructed her to rent a bubble machine and order an ice sculpture.
The senator would call from Harrisburg "several times a day" to make sure certain tasks for the party were completed, Hall said.
In Washington's defense, Hockeimer called Monica Pope, who began working in the senator's Philadelphia office in September 2012. Pope said she has never seen campaign work done in the office or witnessed hostile behavior by the senator.
Another ex-staffer previously told the grand jury that Washington shut him down when he questioned the campaign work on taxpayer-funded time.
"I am the f-ing senator, I do what the f- I want, and ain't nobody going to change me," he told the grand jury, according to its report.
Pope testified Wednesday that she had never heard Washington use such language.
Hockeimer has also questioned the timing of the charges against Washington, which came just days before an Inquirer investigation revealed that Attorney General Kathleen Kane had shut down a sting in which four Democratic legislators were allegedly caught on tape accepting cash or money orders from a lobbyist.
Kane has pointed to Washington's case as proof she does not shy away from prosecuting Democrats.
In a letter to Kane, Hockeimer requested documentation of when her office first became aware of The Inquirer story.
"We believe we are entitled to this information in order to assess whether the nature and timing of the charges against Sen. Washington were motivated by any improper considerations," he wrote.
Washington will be arraigned May 7 in Common Pleas Court.