 -- Newly declared presidential candidate Sen. Bernie Sanders said today he hopes to lead a "political revolution" for working families and against money in politics in his bid for the White House.

"I think I'm the only candidate who's prepared to take on the billionaire class," Sanders, I-Vt., told ABC's George Stephanopoulos on "This Week." "We need a political revolution in this country involving millions of people who are prepared to stand up and say, enough is enough, and I want to help lead that effort."

Sanders, who will run in the Democratic primary against Hillary Clinton, told ABC's Jonathan Karl earlier this week the millions of dollars flowing into the Clinton Foundation poses a "very serious problem."

"It's not just Hillary. It's the Koch Brothers. It is Sheldon Adelson," he said, referring to billionaire backers of conservative causes and candidates. "Can somebody who is not a billionaire who stands for working families actually win an election?"

Sanders could challenge Clinton from her left. He opposed the Iraq War, which Clinton supported in the Senate, and is against Trans-Pacific Partnership trade agreement, which the Obama administration is trying to get through Congress.

Clinton fully supported TPP as secretary of state but has raised reservations about it since announcing her presidential bid.

"Hillary Clinton has been part of the political class for many, many years," Sanders said. "I respect her and I like her, but I think what the American people are saying, George, is ... maybe it's time for a real political shakeup in this country."

He has raised more than $1.5 million since announcing his campaign on Thursday, but has pledged not to have a Super PAC that could accept unlimited contributions.

A self-described socialist who won his first election to become mayor of Burlington, Vermont, by just 10 votes, Sanders has a message for his doubters.

"Very few people thought that I would beat an incumbent Republican to become United States congressman from Vermont by 16 points," Sanders said. "And people weren't so sure I could beat the richest person in Vermont to become a United States senator.

"Don't underestimate me," he added.