ACORN's top officer on Sunday dodged repeated calls to come before Congress and testify about the embattled group's finances and ties to other organizations. 

Rep. Darrell Issa, R-Calif., ranking Republican on the House Oversight and Government Reform Committee, had pressed chief organizer Bertha Lewis to prove her commitment to reforming the community activist group by showing more transparency. 

"The bottom line is there's no transparency in ACORN," Issa said on "FOX News Sunday." 

The pressure comes in the wake of controversy over a series of hidden-camera videotapes showing the organization's employees offering advice to undercover filmmakers posing as a pimp and prostitute. ACORN has pledged to investigate its offices and workers. 

"Internally, let's have some reform," Lewis said. "It's indefensible what I saw (in the tapes)." 

But she refused to answer Issa's request to come before his committee. 

Issa challenged Lewis to provide full disclosure to Congress on the "firewalls" in place to ensure that taxpayer dollars are not going toward political activities. 

"There is no God-given right for any organization to receive a grant from the American people. The fact is there are organizations standing in line that wish they won instead of you, and they're giving us the transparency so we can have the confidence the money is spent only for the purpose of the grant," Issa said. 

Lewis said her organization has "firewalls" to prevent non-political money from going toward political purposes, but Issa said that's not true. 

"You shouldn't get another penny of federal dollars until you demonstrate that those dollars are firewalled for only that use, and that has not been the history of the organization," Issa said. 

Pressed again on whether she would open her books to Congress, Lewis said: "I am willing to do the work that I need to do every single day, not be distracted, make sure that things that we do well we beef up, and things that we don't do so well that we change and we reform." 

Lewis, meanwhile, was trying to calm a growing firestorm over the behavior of her group's employees. Allegations of voter registration fraud and other ethical lapses have plagued ACORN for years, but the videotapes were the last straw for the group's critics -- as well as its long-time defenders. 

The videotapes, filmed by two conservative activists, led to both the House and Senate voting to de-fund ACORN last week. Many Democrats joined Republicans in voting for the measures. 

Lewis initially took a defiant tone against the criticism of her organization when the first tapes were made public by filmmaker James O'Keefe. But that stance softened as more tapes were revealed, detailing hidden-camera operations at five separate locations across the country. 

"Any organization is not entirely perfect," Lewis said on "FOX News Sunday." "I was outraged by it. Everyone should be, and I can understand how the Congress was also." 

She said any employee "too stupid" not to adhere to professional standards will be terminated. 

President Obama, in an interview broadcast Sunday, said he favors an investigation into ACORN. At the same time, he played down the controversy, saying it is not the "biggest issue facing the country." 

The ACORN fallout came after employees in the secret videotapes were seen offering advice on how O'Keefe and his partner could skirt anti-prostitution and tax laws -- by lying about their occupation, claiming underage sex workers as "dependents" and using other means of deception. 

Soon after the controversy erupted, Lewis released a statement suggesting race was behind an attempt by conservative critics to "destroy" ACORN. ACORN provides assistance to low-income and often minority communities. 

Lewis claimed the tapes were "doctored" and threatened to sue FOX News -- which broadcast the tapes but did not produce them -- and others. 

But after Lewis claimed O'Keefe tried the same "scam" in many other locations and failed, O'Keefe continued to release more tapes in more locations showing ACORN workers giving him and his partner helpful advice. 

Last week, Lewis reversed. She said ACORN would conduct an internal review and put a halt on much of the group's community activity. 

Lewis said on "FOX News Sunday" that the group would name an auditor Monday.