Bernie Sanders Responds to Possible Joe Biden Run

The Democratic candidate spoke with ABC News' Jonathan Karl

Sanders made the statements in response to a question from ABC News' Jonathan Karl about Biden considering another run for president.

Sanders is the longest serving independent member Congress, but he is seeking the Democratic nomination for president. The 73-year-old has mobilized an impressive grassroots campaign since announcing his candidacy in May. Last week, more than 100,000 people RSVP’d to attend one of 3,700 events across the country, but his support continues to be concentrated in liberal, urban centers.

Sanders defended his campaign, saying it will be able to expand its support to a wider and more diverse base across the country.

“We have made phenomenal progress in the last three months, and we are going to continue to make that kind of progress,” he said.

According to a Quinnipiac poll released last week, 57 percent of registered voters do not think she is trustworthy. “I have a lot of respect for Hillary Clinton,” Sanders said. “I am not going to be engaging in personal attacks against her.”

He did concede, however, that he “had a hard time understanding” her refusal to take a position on the Keystone pipeline this week. Sanders has helped lead the opposition against the project.

Republican contender Donald Trump, however, did take the opportunity to attack Hillary Clinton, adding that Vice President Joe Biden could possibly topple her for the nomination.

“I think she's got a big problem with the emails and obviously her numbers are going down drastically, so somebody like Biden could probably go in and do very well and maybe win," he said.