Vice President Biden insists he's thinking about running for president. As we've written before, this is probably a fool's errand.

Yes, Democrats like Biden just fine. In fact, a new Bloomberg/Des Moines Register poll of the Iowa caucuses shows Biden's popularity among likely Democratic caucus voters is almost as high as President Obama's and Hillary Clinton's (at least on its surface). While Obama is viewed favorably by 86 percent of these folks and Clinton is at 84 percent, Biden is at least in the same ballpark, at 78 percent.

What's more, Biden is the first or second choice of 35 percent of likely Democratic caucus-goers when it comes to the 2016 contest.

But to truly understand just what Biden's 2016 problem is, you have to go a little deeper. Yes, His favorable rating among Iowa Democrats is 78 percent, but the picture for Biden is much less promising when you break that number down. Of that 78 percent, 53 percent say they have a "mostly" favorable view of him; just 23 percent say they have a "very" favorable view.

By comparison, Clinton's "very" favorable number is 46 percent and Obama's is 49 percent. Even Sen. Elizabeth Warren (D-Mass.), whose name ID is considerably lower than Biden's, has 30 percent of likely Iowa Democratic caucus-goers who view her "very" favorably.

Such is Biden's problem. He's a likable enough guy for Democrats, but he's not the guy they see as their 2016 standard-bearer and a future president. He's not the guy who's getting anybody fired up about the 2016 campaign.

We got at this a little a while back. A Quinnipiac poll at the time asked people whether they thought Biden would make a good president. Just 51 percent of Democrats and 19 percent of independents agreed with that statement. Thirty-eight percent of Democrats answered in the negative.

This same pollster, we would note, has shown Biden's favorable rating among Democrats about where it is in the Iowa poll -- 73 percent favorable vs. 18 percent unfavorable in June 2014. Again, a popular guy, but not one who is lighting fires in the hearts of voters.

Why does this matter? Well, if Clinton runs for president, it probably doesn't. That's because neither Biden nor anybody else would probably have a great shot to beat her.

But even when you're talking about the small chance that an alternative would have -- or Biden's odds were Clinton to bypass a run -- these numbers suggest he would have a very difficult time being that second option for Democratic voters.

He's just a little too "Uncle Joe" for his own good.