INDIANOLA, Iowa -- Sen. Kamala Harris on Thursday clarified her position on federally mandated school busing, saying it's only necessary in cases where local governments are actively opposing integration.

Harris said that in the 1960s and '70s, institutions "were literally working against integration of our schools." That's why she supported busing then, she said, but now thinks it should just be a "tool" available to local governments and school districts to address segregation.

"Today it is very rare that we require the courts or the federal government to intervene," Harris told reporters Thursday before a campaign event in Indianola, Iowa.

Her stance on busing came under scrutiny after last week's debate, when she went after former Vice-President Joe Biden for his stance on busing while he was a senator. Biden said he was in favour of voluntary busing but opposed federally mandated busing. During the 1970s and '80s, however, Biden actively worked against busing efforts and was an outspoken critic of the tactic.

But Harris muddied the waters Wednesday when she told reporters she too did not support federally mandated busing and supported it only as an option for local governments.

On Thursday, Biden told reporters after seeing her latest comments that he believed the two agreed on the issue, and that he felt her critique had come out of nowhere.

Harris, however, told reporters "we do not agree," pointing to his past criticism of busing, and said she was surprised Biden was caught off-guard by her debate stage criticism.

"Part of the impetus of the conversation was the statements that the Vice-President made about his work with segregationists and that was the subject of conversation for days on end, so if he and his team weren't prepared for the topic I don't know what to say about that," she said.

The feud erupted as Harris enjoyed a surge in a number of post-debate polls and both she and Biden descended on Iowa for the Fourth of July holiday. Iowa is a key state for both candidates as they vie for the Democratic nomination.