Washington, through a spokesman, refused to comment for this article.
Her opponents say Washington's absence is nothing new.
As a Cheltenham Township commissioner, Art Haywood said, he noticed that "our state senator was not effective and not responsive to the needs in our community."
"We need a state senator who is going to be active, not miss meetings, not going to vote for [school] vouchers or miss a vote on voter ID," Haywood said. He was referring to an Inquirer investigation that found that Washington missed nearly a third of the 2012 and 2013 legislative sessions.
"She's not visible in the district, nor is her staff," said another contender, Brian Gralnick, a community organizer who is making his first run for public office. "I've been very active in this community over 10 years, and rarely have I seen her."
Washington, 68, was elected to the Senate in a special election in 2005. Prior to that, she served six terms in the state House.
Before being elected to local office, Haywood, 57, worked as an attorney for housing and community development projects in Northwest Philadelphia. His wife is on the Cheltenham Township school board.
Gralnick, 34, has worked in nonprofits and in the Department of Aging under Gov. Ed Rendell.
He now directs the Center for Social Responsibility at the Jewish Federation of Philadelphia.
On most issues, there is little difference among the candidates. Both Gralnick and Haywood have made public-school funding a centerpiece of their campaigns.
Gralnick is also emphasizing an end to mass incarceration and increased services for seniors, while Haywood is focusing on gun control and raising the minimum wage.
Gralnick often speaks in lists, rattling off accomplishments and wonky policy priorities as he stumps the district.
At a senior home in Elkins Park last week, Gralnick was already providing constituent services.
"Let me get you that number. Hold on," he said, scrolling through his cellphone to answer a woman's question about elder abuse.
Haywood jumped into the race last spring, before Washington's indictment. He has been campaigning nearly every day, with phone banks, door-knocking, and letter-writing campaigns.
At a Haywood campaign party in Elkins Park in April, a diverse group of supporters milled about and admired the Obama "Hope" replica painted on the wall of Haywood's campaign headquarters.
"He's the best person in the race," said State Rep. Dwight Evans, the veteran Philadelphia Democrat who started working with Haywood 20 years ago on community projects around Ogontz Avenue. "He's always talking about housing and development, and he has taken that skill set over to being a commissioner in Cheltenham."
The district covers Mount Airy, Germantown, Ogontz, Northwest Philadelphia, Abington, Springfield, Cheltenham and Jenkintown. But all four candidates - including the Republican, registered nurse Robin Gilchrist - live in Cheltenham.
The district is nearly 75 percent Democratic, and the population is split evenly between Philadelphia and Montgomery County.
Unseating an incumbent - even one with low name recognition and legal troubles - will be an uphill battle, experts say.
"There is a Democratic establishment, and that establishment has been aligned behind the incumbent for 20 years or more," Haywood said.
Andrea Putnam, Gralnick's campaign manager, said their outreach has found that most people do not know who their state senator is - let alone anything about her record.
The Philadelphia City Democratic Committee is standing behind Washington, as are many of her colleagues in the Senate Democratic Caucus. But two of the seven Philadelphia wards have split off to endorse her opponents, the Ninth Ward backing Gralnick and the 10th backing Haywood.
The Montgomery County and Cheltenham Township committees voted to remain neutral.
Haywood has Evans endorsing and campaigning for him, while Gralnick last week was endorsed by the Pennsylvania chapter of the National Organization for Women.
Both challengers said they were optimistic about their chances. But Haywood conceded that, with two well-qualified opponents in the race, "it is possible that the incumbent could get 40 percent of the vote, 60 percent against her, and win."