According to local reports, Harkin also said Biden "has a long history of working with Iowa Democrats, so I think that he would find - how should I phrase it? - fertile ground in Iowa if he should choose to run."
On Sunday, Harkin said the state has "endured" visits by a crop of potential Republican presidential candidates, including former Sen. Rick Santorum, Sen. Rand Paul and Sen. Ted Cruz, with "Governor 'Oops' Perry on his way."
"All I can say, folks, is the clown car is filling up pretty rapidly, early in the season," he joked.
A contrast in generations
Harkin said Castro and Biden represented the "new and the seasoned" that are needed in the Democratic Party.
In his speech, Castro talked about believing in the "blueprint" laid out by Obama and Biden, especially on health care, and emphasized the importance of education. He said his grandmother didn't get to finish school and had to work in the fields.
Saying this was his first real trip to Iowa, Castro said he looked out the window on the plane and took note of the "all of the fields, the green space."
"And I thought how proud she would have been that she had been picking crops and her grandson would be here where you guys pick a president of the United States," he said.
He encouraged Iowans to get out the vote for Rep. Bruce Braley of Iowa, the only announced Democratic candidate for next year's Senate race. Braley also has strong support from Harkin.
"I look forward to Election Night November 2016, I'll be in San Antonio, I'll probably have the TV on," he said. "I look forward to watching CNN and CBS and MSNBC and ABC. And I look forward to the moment when Brit Hume on Fox News turns to Bret Baier and says 'Bret, we're calling Iowa's electoral votes for the next president of the United States, the Democratic nominee for president."