“There are real crises facing this country. When I give a speech, often it’s 45 minutes or an hour, OK? Because there are a lot of challenges that the country faces, and I’ve got to talk about them,” Sanders continued, before launching into a critique of Biden.

“You know, I think Joe was somewhere — where was he, I don’t know — Michigan or some place else the other day, and he spoke for seven minutes. I don’t know how you say anything other than, you know, minimal discussion in seven minutes,” Sanders said.

Sanders’ supporters and senior staff blasted Biden over the weekend for a series of Missouri campaign events where the Democratic frontrunner offered abbreviated speeches, speaking for roughly seven minutes in St. Louis and around 15 minutes at a subsequent stop in Kansas City on Saturday.

“All I would say is, I’m not here to criticize Joe, but to say that I think the American people in this incredibly complicated and difficult moment in our history are entitled to thoughtful answers to the crises we face,” Sanders said on Monday.

Pressed on whether he was asserting that Biden’s public statements lacked substance, Sanders appeared to double-down on his previous response and argued that presidential candidates “owe respect to the people” when making the case for their campaigns.

“Well, I’m just saying, look, I’m here. We’re talking about real issues, alright? And we can go on for three hours talking about them,” Sanders said. “If I’m going to give a speech to people, and that was at a rally or an event that he held, I can’t do it in seven minutes.”

The jostling between Biden and Sanders has intensified ahead of the slate of state primaries taking place on Tuesday, including salvos by the senator against his competitor’s his record on Social Security, vote to authorize the Iraq War and support for trade deals including the North American Free Trade Agreement.

But the recent sparring has also evoked concerns regarding the two septuagenarians’ readiness to assume the highest office in the land, with Sanders calling for the upcoming primary debate on Sunday to feature the pair of candidates standing at lecterns and accusing Biden of wanting to sit down.

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Meanwhile, President Donald Trump and his Republican allies have sought to cast Biden as an elderly, senile politician prone to frequent verbal stumbles in the Democratic nominating contest and unprepared for the rigors of the Oval Office.

Even some of Biden’s former primary competitors have suggested he may not be up to the challenge of taking on Trump in a grueling general election campaign. Former Housing and Urban Development Secretary Julián Castro appeared to question Biden’s mental acuity at a September debate, while New Jersey Sen. Cory Booker claimed voters were worried about his “ability to carry the ball all the way across the end line without fumbling.”

But after being played clips of those comments by Castro and Booker on Monday, Sanders declared that he was “not going to go to that level in attacking” Biden as their fight for the nomination continues.

“You know, that’s for people to decide,” Sanders said. “All I can say is Joe and I have very significant political differences. And I’m not going to be making, you know, personal attacks on Joe. That’s not what I do.”