Harkin, who will retire from the U.S. Senate after completing his fifth term in 2015, is keeping his options open while continuing to dangle his famous "steak fry" fundraiser as a king- or queen-making opportunity.
Biden will headline Harkin's steak fry Sunday. And even though there was considerable interest by reporters who wanted to travel with him, they were put off by the jaw-dropping $2,900 cost and all decided to fly commercial.
But why would Biden accept the invitation to such a high-profile political event if he were not toying with running for president? Unless he just wants to drive Clinton crazy.
Harkin likes Joe, and Harkin likes Hillary -- though he has certain caveats for both.
"Biden can't run just by saying, 'I've been vice president,'" Harkin says.
And Hillary's problem?
"The biggest mistake a politician can make is if they think they deserve something, if they think the party is going to hand them something," Harkin says. "People vote on what a person is going to do tomorrow."
It is no secret that Hillary has an Iowa problem. She didn't seem to enjoy running here in 2008, when she came in second to Barack Obama, or in any other caucus state, where organization is very important.
Of course, when Biden ran here in 2008, he got 0.9 percent of the delegates and dropped out. So why is Biden coming here?
"Joe Biden loves Iowa," Harkin says. "He wants to make sure he keeps his contacts up here. He's got a lot of friends here. I think Iowans were very happy Obama picked him for vice president."
Biden would be 74 on Inauguration Day in 2017, and Clinton would be 69, which Harkin, 73, doesn't see as a problem.
"I think (2016) is going to be an election where people, because of the times, want someone who has been tested, someone who has the maturity to be president," Harkin says.
Harkin believes that 2016 will be determined by the results of the 2014 congressional races, the economy and the "fragmentation" of the Republican Party. "Will the Republicans nominate a Ted Cruz (the junior senator from Texas) or Rand Paul (the junior senator from Kentucky) or a more moderate person, like Chris Christie (governor of New Jersey)?" Harkin asks.
Harkin believes that the Republicans are headed away from moderation, which will lead them to 2016 results "that will be worse than the Barry Goldwater debacle of 1964."
Harkin says that if unemployment is 5 percent or lower, if the deficit goes down and if "there are no more foreign wars," then the Democrats ought to win the White House in 2016.
Unless they screw it up.
"The base has to be passionate about you," Harkin says, "but you can't be hard-left."
And in Democratic terms, neither Biden nor Clinton, each with close ties to the Obama administration, would be considered hard-left.
"Hillary, like Joe Biden, would be a formidable candidate," Harkin says. "Will she run? I honestly don't know." But "there are pressures and groups so insistent" that Hillary run, Harkin says, that she may feel she has to.
But would she make a good candidate?
"I thought she was a good candidate the first time," he says. "She just was a lousy organizer." He pauses. "I shouldn't say that. Let's just say she got out-hustled, out-organized by Obama."
But 2016 would be different. "One thing I know about the Clintons is that they learn," Harkin says. "And she'd have another dimension by virtue of her experience with foreign affairs. And she might have a more populist appeal than in the past."
On the other hand, Harkin says, the party may turn to someone new, such as San Antonio Mayor Julian Castro, who also will be at the steak fry, or Maryland Gov. Martin O'Malley or New York Gov. Andrew Cuomo. Or Sen. Amy Klobuchar of Minnesota. Or Sen. Kirsten Gillibrand of New York.
Harkin doesn't want to rule anybody out.
But, admit it, Joe vs. Hillary would be a pretty cool battle, wouldn't it?
"Oh, my gosh. Oh, man, that would be a good contest," Harkin says. "That would get a lot of juices flowing."
(Daily Corinthian columnist Roger Simon is Politico's chief political columnist.)