Donnelly denies the accusations in the lawsuits, and he said he knew nothing about the attorney general's investigation.
If McCauley succeeds, he'll be the first Democrat to unseat a Republican row officer in Bucks in decades.
And that has some wondering whether local contests there will reflect the competitive nature of the county, which is considered a national battleground.
"The real question is, is this an election where the Democrats can make big gains in the row offices?" asked G. Terry Madonna, the veteran political scientist and pollster at Franklin and Marshall College.
"There's probably an under story here in the Democrats" chances, said Charlie Gerow, a longtime Pennsylvania GOP consultant.
Pat Poprik, chair of the Bucks County Republican Committee, said Democrats would continue to "grab at straws" to win countywide offices, which on Tuesday will include sheriff, prothonotary, recorder of deeds, and controller.
District Attorney David Heckler, a Republican, is running unopposed.
McCauley has been on the offensive for months, hammering Donnelly in the press and on social media.
One of his criticisms is the attorney general's investigation.
In 2011, Heckler's office launched a probe into the Sheriff's Office but turned it over to the attorney general, citing a potential conflict of interest. Neither Heckler nor a representative of the Attorney General's Office would discuss the nature of the probe. No charges were filed.
A spokesman for the state prosecutor's office this week said he could not confirm or deny an investigation.
McCauley also references the pending lawsuits filed by sheriff's deputies fired last year for failing to report a sergeant's assault on a handcuffed suspect.
One of the deputies alleged that Donnelly had him plant campaign signs during the last election, a charge Donnelly denies.
And McCauley criticizes Donnelly for accepting campaign donations from his deputies despite Donnelly's assertion that he doesn't talk about fund-raisers at work.
"The Sheriff's Office is broken, and it needs to be fixed," McCauley said. "The organization needs the kind of leadership that I can bring to it."
McCauley is director of security and safety at Bucks County Community College, overseeing 25 officers there. A former division commander in Abington, he said he would bring management skills to the Sheriff's Office.
He also said he would eliminate "the toxic atmosphere of politics" in the office, serve only two terms, and improve the office's website and social media presence.
Donnelly, who won the 2009 election by 20,000 of the nearly 100,000 votes cast, defended himself and took pride in his office in an interview this week.
He said he would concentrate on continuing to "professionalize" the department following its accreditation in June from the Pennsylvania Chiefs of Police Association. The distinction recognizes offices for training and procedural standards.
"Our department worked hard for that," Donnelly said, noting that it was only the fourth sheriff's office in the state to earn the recognition.
Donnelly did not fire back at his opponent.
Poprik, the GOP chair, said she doubted anyone would care about the allegations of two fired employees, who she said have an agenda, and an investigation that has produced no charges.
"I just don't think the public is duped by that stuff," she said. "The voters in Bucks County are very intelligent. They can see through it."
John Cordisco of the Bucks County Democratic Committee said that turnout would be the most important factor Tuesday because the county has enough registered Democrats to win.
According to the Department of State, Democrats outnumbered Republicans by 186,101 to 174,527 through Oct. 21.
On the issues, Cordisco said, McCauley should win. "You have a cloud over that office," Cordisco said, "and at the same time an exceptional candidate."