DOVER — A former chief of staff to Delaware State Auditor Kathy McGuiness admitted during testimony in her criminal corruption trial Thursday that he lied under oath to a grand jury that was investigating McGuiness.

Thomas Van Horn acknowledged under cross-examination by the defense that he told the grand jury in April 2021 that Ms. McGuiness had fired him a month earlier because of “disagreements” they had.

In fact, Ms. McGuiness asked Mr. Van Horn to resign because he had been in a sexual relationship with a subordinate staffer.

Mr. Van Horn also acknowledged that he never volunteered information to prosecutors and investigators about his relationship with Rachael King, who also is expected to testify. Instead, he claimed that he was terminated because Ms. McGuiness had heard a “potential rumor of a sexual relationship” with someone in the office, and that he might be a victim of “defamation.”

“When you said it was a rumor, that was a lie, because it was a true story, wasn’t it?” defense attorney Steve Wood asked Mr. Van Horn.

“Yes,” replied Mr. Van Horn, who moments later contradicted himself and denied any sexual relationship.

“What is the definition of a sexual relationship?” asked Mr. Van Horn, who in earlier testimony agreed with prosecutor Mark Denney that his former relationship with King could be described as “romantic.” In a recorded interview earlier this year, he told Mr. Denney that he and Ms. King “made out here and there” but never had a sexual relationship.

Mr. Wood’s cross-examination came after Judge William Carpenter Jr., outside the jury’s presence, held a long private conversation with attorneys before reminding Mr. Van Horn that he was under oath and required to tell the truth.

“I assure you sir you do not want to mess with this judge if you’re lying,” Judge Carpenter told Mr. Van Horn sternly.

Ms. McGuiness, a Democrat elected in 2018, is responsible as state auditor for rooting out government fraud, waste and abuse. She is being tried on felony counts of theft and witness intimidation, and misdemeanor charges of official misconduct, conflict of interest and noncompliance with procurement laws.

Prosecutors allege, among other things, that Ms. McGuiness hired her daughter and her best friend as temporary employees in 2020, even though other temporary employees had left because of the lack of available work amid the coronavirus pandemic. Authorities allege that in hiring her daughter and exercising control over taxpayer money with which she was paid, Ms. McGuiness engaged in theft of state money and conflict of interest.

Prosecutors also have alleged that Ms. McGuiness orchestrated a no-bid “communications services” contract for My Campaign Group, a firm she had used as a campaign consultant when running for lieutenant governor in 2016. They also contend that she deliberately kept the contract payments under $5,000 each to avoid having to get them approved by the state Division of Accounting. The owner of My Campaign Group, Christie Gross, successfully bid on a second contract after forming a new company called Innovate Consulting. Ms. Gross has testified that Ms. McGuiness never told her to submit a bill for less than $5,000. She also described the auditor’s office as a “disaster” that “lacked professionalism,” and testified that she had urged Ms. McGuiness to fire Mr. Van Horn immediately after learning about his relationship with Ms. King.

In the fall of 2020, Mr. Van Horn approached deputy attorney general Patricia Davis, who was assigned to the auditor’s office, with concerns about how the office was being run. Ms. Davis suggested that he call Frank Robinson, an investigator with the Division of Civil Rights and Public Trust.

Mr. Van Horn said he approached Ms. Davis after Ms. McGuiness told him to use his state credit card to pay off the overdue final balance on the My Campaign Group contract, which had exceeded the initial budget allocation of $45,000. Mr. Van Horn said Ms. McGuiness stood over his shoulder while he made the Pay Pal transaction, making him “uncomfortable.”

In a November 2020 call with Mr. Robinson, however, Mr. Van Horn never mentioned the credit card incident. He instead made vague references to contracts entered into by Ms. McGuiness with a “political person” and complained that the office was undertaking a lot of “special reports” on policy issues that were taking away from auditing work it is required to do.

He also complained to Mr. Robinson that a request for proposals that led to Ms. Gross successfully bidding on the contract awarded to Innovate was drawn up by a young staffer, rather than a veteran staffer who normally handled such duties. Van Horn testified Thursday that he was pressured by Ms. McGuiness’ executive assistant to score Ms. Gross’ bid higher than another firm’s competing bid, but he never mentioned that in his call to Mr. Robinson.

Mr. Van Horn also testified that he never interviewed Ms. McGuiness’ daughter, Elizabeth, for the part-time job she began in May 2020, contradicting the daughter’s testimony from Wednesday.

At 4:30 p.m. Thursday, Judge Carpenter asked attorneys for the estimated time needed to present the rest of their witnesses. Mr. Denney said a best-case scenario for resting would be mid-morning Monday. Mr. Wood said he expected that he would need 2 to 2 1/2 days to make his presentation, and witnesses would include Department of Justice employees.

Judge Carpenter indicated he was concerned about the length of the jurors stay at trial. He said defense and prosecution attorneys would be limited to 90 minutes for closing arguments.

Testimony resumes today.