Greensboro, North Carolina (CNN) -- John Edwards' defense team hammered away at a former campaign aide on Thursday after he was quizzed about his motives and asked whether he made up stories about how Edwards allegedly concealed contributions from campaign donors.
On day four of the trial, Andrew Young, the former aide, said "we lost our perspective" during cross examinations, acknowledging using donations for personal expenses at a California home. Young, testifying in U.S. District Court in Greensboro, North Carolina, said he used the funds in constructing a pool and a theater.
Young is considered the government's star witness against Edwards, a former Democratic senator and presidential candidate from North Carolina who is accused of using hundreds of thousands of dollars of donations to conceal his affair with a campaign videographer without reporting the money to federal authorities.
The money was allegedly used to hide his pregnant mistress from the public so he could continue his 2008 presidential bid.
Wednesday was the first day the defense cross-examined Young, and they focused mostly on apparent inconsistencies between Young's public statements and things he wrote in a book about the case.
For example, Young has described a time when Edwards returned home from a trip overseas with a phone purportedly given to him by his mistress, Rielle Hunter, the videographer. Young said Edwards was asleep when the phone rang, and Edwards' wife, Elizabeth, answered.
Hunter began talking, assuming John Edwards was on the other end of the line.
Under cross-examination, Young admitted he had no firsthand knowledge of the story.
Defense attorney Abbe Lowell also questioned Young's assertion that Edwards refused to take a call from Hunter as she left for the hospital to have Edwards' baby, something Young described as "bone-chilling" in its disregard. Lowell pointed out Hunter later said Edwards had, in fact, called her.
Prosecutors say Edwards broke federal law by accepting about $725,000 from 101-year-old heiress Rachel "Bunny" Mellon and more than $200,000 from another donor, Fred Baron, a now-deceased Texas lawyer who was his finance chairman.
The money was used to pay for Hunter's living and medical expenses, travel and accommodations to keep her out of sight while Edwards made his White House run, prosecutors say.
Edwards is accused of concealing the money from the public and the Federal Election Commission, which polices political contributions, by filing false and misleading campaign disclosure reports.
Young is testifying for the prosecution under a cooperation agreement with the hope that he won't be prosecuted.
He testified Tuesday that he let Hunter move in with him and his wife at Edwards' request, after newspapers began looking into a possible affair within the Edwards campaign.
Wednesday, Young testified that the two donors, Mellon and Baron, were not only aware of the living arrangements but also funded Hunter's expenses while she lived there.
Mellon was already funding Hunter's expenses when Young called Baron and complained about the situation, according to testimony Wednesday. Baron offered to help out, telling Young to write up Hunter's expenses so Baron could reimburse them.
Neither Baron nor Mellon appeared to know that the other was reimbursing Young for the same expenses, raising questions about whether and how much Young may have profited from the situation.
Hunter eventually moved to a gated community and the money continued to be used to pay her expenses.
Invoices for those alleged expenses, shown in court Wednesday, reveal a lifestyle many would consider expensive: $37,700 for a rental house for the year, more than $28,000 for a car, $40,000 for cash expenses and nearly $25,000 in other expenses.
The defense argues the money Edwards received from Mellon and Baron was for personal reasons: to protect Edwards' wife, Elizabeth, who was dying of cancer, and his family from public humiliation. Edwards has said his actions were wrong but insisted they were not illegal.
Edwards could face up to 30 years in prison if convicted of all six felony and misdemeanor counts against him.
Young is the author of the tell-all book "The Politician: An Insider's Account of John Edwards's Pursuit of the Presidency and the Scandal That Brought Him Down."
Young testified on Wednesday that he wrote the book for two reasons: "It had been almost three years since Mr. Edwards promised me he would tell the truth (and) we also very much needed the money."
Lowell, however, repeatedly asked Young on Wednesday whether he "made up a story" while writing the book.
Lowell produced a February 2009 e-mail between Young and a man named Robert Draper, who helped with the book, in which Young wrote, "I want to personally s**t on his head."
With Young, Lowell painted a picture of a man scorned by someone he greatly admired.
Asked whether he "fell in love with John," Young said, "We all did."
"You fell out of love with him?" asked Lowell.
"Yes," replied Young.
CNN's Adam Reiss contributed to this report.