Nine months after leaving the Obama administration, Hillary Clinton sat on a stage under the life-size model of a blue whale that hangs in the American Museum of Natural History.

For a fee of $275,000, she had agreed to appear before the clients of GoldenTree Asset Management, the capstone of a lucrative speechmaking sprint through Wall Street that earned her more than $2 million in less than seven months.

Mrs. Clinton said the Dodd-Frank rules, while unpopular among some on Wall Street, were a necessary response to the financial crisis, according to one person who attended, while making clear she viewed Wall Street as a partner in securing the country’s economic future, not an enemy. We have to win together, she said, not divide ourselves.

But her paid speeches are now emerging as the central line of attack in an increasingly bitter primary clash with Senator Bernie Sanders of Vermont. In Sunday’s debate in South Carolina and at a series of campaign appearances in Iowa this week, Mr. Sanders has argued that Mrs. Clinton is too personally beholden to Wall Street to effectively rein in the industry’s excesses.