Former Obama advisor David Axelrod expressed his concerns that former Vice President Joe Biden is "playing into the the caricature" made by President Trump that has questioned the Democratic frontrunner's fitness to lead.

Both Trump and Biden exchanged harsh attacks between one another on Tuesday as they both made campaign stops in Iowa, with the president telling reporters outside the White House that the former VP is "weak mentally.”

Appearing on CNN, Axelrod cautioned that Biden's light campaign schedule, when compared to those of other competitors in the Democratic field, left him open to such criticisms.

"The thing that I think is important for Biden is to run an energetic campaign that dispels any of these caricatures," Axelrod told CNN anchor Anderson Cooper. "He hasn't had a very heavy campaign schedule. He has sort of played into the caricature and I think he is going to have to come forward... with ideas and he going to have to campaign energetically."

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Axelrod, who is now a CNN host and political analyst, explained that ahead of the Iowa Caucus, people "expect to see you" and "want to interact with you," stressing that candidates "have to be there."

"You can't do drop-bys and use them as a backdrop to attack the president," Axelrod added. "You got to do much more than that so I think that's very important."

The comments came after Biden returned fire at Trump Tuesday.

In what could be a preview of next year’s general election campaign for the White House, Biden is set to blast the Republican president for threatening tariffs on global trading partners. Warning of the impact it’s had on American farmers and manufacturers, Biden plans to call Trump an “existential threat” to the nation.

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“You know, Donald Trump and I are both in Iowa today. It wasn’t planned that way, but I hope Trump’s presence here will be a clarifying event,” Biden will say during an evening speech in Davenport, according to prepared remarks.

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The former vice president’s remarks, released early Tuesday morning by his campaign, contain more than 40 mentions of Trump by name, including some of Biden’s most blistering attacks to date on the president.

Fox News' Paul Steinhauser contributed to this report.