Christine O'Donnell, who failed in her bid to win a seat in the U.S. Senate, is being probed over her use of campaign funds. (Manuel Balce Ceneta/Associated Press)

Failed U.S. Senate candidate Christine O'Donnell said Thursday that accusations she misspent campaign funds are politically motivated and stoked by disgruntled former campaign workers.

The Delaware Republican appeared on several network morning shows to defend herself a day after The Associated Press reported federal authorities have launched a criminal probe to determine whether she broke the law by using campaign money to pay personal expenses.

"There's been no impermissible use of campaign funds whatsoever," O'Donnell told ABC's Good Morning America.

O'Donnell, the Tea Party favourite who scored a surprise primary victory before losing in the general election, suggested the accusations are driven by political establishments on the right and left, including Joe Biden. He represented Delaware in the Senate for decades before he became vice-president.

"You have to look at this whole thug-politic tactic for what it is," she said Thursday.

O'Donnell said she found it suspicious that she, her campaign staff and her lawyer have not been informed of a federal investigation.

A person familiar with the investigation confirmed it to The AP, speaking on condition of anonymity to protect the identity of a client who has been questioned as part of the probe. The case, which has been assigned to two federal prosecutors and two FBI agents in Delaware, has not been brought before a grand jury.

Dogged by questions about finances

O'Donnell, who set a state record by raising more than $7.3 million US in a Tea Party-fuelled campaign this year, has been dogged by questions about her personal and campaign finances.

At least two former campaign workers have alleged that she routinely used political contributions to pay personal expenses including her rent as she ran for the Senate. She has run three consecutive times, starting in 2006.

O'Donnell has acknowledged paying part of her rent with campaign money, arguing that her house doubled as a campaign headquarters.

On Thursday, she told NBC's Today Show that people making the spending allegations include a fired former staff member and a former volunteer, both of whom she described as disgruntled. She said many other workers who spent longer with her campaigns have defended her.

Her contention that the accusations were politically motivated echoed a written statement she released the day before, which singled out Biden.

"Given that the king of the Delaware political establishment just so happens to be the vice-president of the most liberal presidential administration in U.S. history, it is no surprise that misuse and abuse of the FBI would not be off the table," she said.

Critical of watchdog

The vice-president's office declined to comment.

O'Donnell's campaign also has criticized the nonpartisan watchdog group, Citizens for Responsibility and Ethics in Washington, which filed a complaint about O'Donnell's campaign spending this fall and asked Delaware's federal prosecutor to investigate.

O'Donnell said the group is part of a liberal effort to kill her career, noting that the organization is run by Washington attorney Melanie Sloan, who worked under Biden as a lawyer for the Senate Judiciary Committee in the early 1990s.