Hillary Clinton's poor approval numbers can be traced back to attacks launched on her during the Democratic primary by her chief rival, Sen. Bernie Sanders, I-Vt., according to one of her most dedicated backers.

"He did do significant damage to Hillary's negatives," Center for American Progress President Neera Tanden said in a recent interview with Politico released Monday.

"I mean, he drove a lot of those negatives, and the truth of it, I mean, just to be candid or honest about it, you know, I think getting those kinds of attacks from another Democrat or another liberal or another progressive is much tougher for Hillary, and it was really hard," she said.

Sanders hit Clinton particularly hard during the primaries over her connections to major Wall Street firms, including the millions of dollars she accepted from giving paid speeches to groups that included Goldman Sachs and Bank of America.

"If you look at her trust numbers the last six months of that primary, it was much, those numbers took a much sharper dive and hard to recover from," said Tanden, who is part of the Clinton transition team leadership.

Though national polling shows Clinton consistently beating GOP nominee Donald Trump, many surveys also show the Democratic candidate has a major trust problem.

A recent CBS News poll, for example, showed Clinton had widened her lead over Trump in Ohio by six points. But the survey, which has a margin of error of plus or minus four percentage points, also showed she is not viewed in Ohio as "looking out for people like you."

Fewer than 44 percent of survey respondents said they think the Democratic candidate is looking out for them, "which also connects to voters' doubts about her truthfulness, and to the fact that many voters feel she is too connected to the influence of foreign donors," the poll found.

Tanden's criticism of Sanders' primary attacks come as the Vermont lawmaker prepares to return to the campaign trail to stump for Clinton.

The senator plans to hit the road shortly after Labor Day, he revealed in a recent interview.

"I look forward to it," he said. "I feel very strongly that Donald Trump would be a disaster for the country. I want to do everything I can to see that Secretary Clinton wins."

The senator is expected to stump for the Democratic candidate in quite a few states, including New Hampshire, Maine, Michigan Wisconsin, Ohio, Pennsylvania and Nevada.