This is also happening in the context of there being essentially two Democrats running for the presidency. There is Hillary Clinton, and there is Not Hillary Clinton. Not Hillary Clinton used to be named Elizabeth Warren; Not Hillary Clinton is now named Bernie Sanders.
This is overly simple, admittedly. Sanders has fans that Warren didn't and vice versa. But Sanders' position in New Hampshire, for example, is basically the same as Sanders-plus-Warren polled in May.
Granted, a new poll was released late Monday night suggesting that Sanders has moved into a slight lead. We'll see if that holds up over time in other polls. The graphs above compare polls from the same pollster.
But back to the crowds. Let's say the Republican field more closely mirrored the Democrats. Imagine there were two Republicans running for president, Jeb Bush and Not Jeb Bush, where Not Jeb Bush did a better job appealing to the outer boundary of the party. Do you think that Not Jeb Bush couldn't roll up 20,000 people at a campaign stop in, say, Houston?
This is the point at which the question invariably turns to the contrast between Clinton and Sanders. Clinton isn't drawing these crowds, the argument goes. Sanders is capturing the energy. Both of which are true points. Sanders, as the underdog and the newcomer, has an energy that Clinton doesn't. She is not drawing the same crowds.
Part of this, though, is a conscious decision. Sanders, coming from the back of the pack, has to show that he's a legit candidate. He needs to be chuffed up. He needs to look big. Clinton has been trying to do the opposite. Her campaign launch was a Sunday e-mail followed by a road trip to Iowa. She's doing her best not to look like a behemoth. Her unofficial launch, on Roosevelt Island, was a neatly tailored group of a few thousand, packed into a space meant to display an audience that size for the cameras.
Could Clinton fill an arena in Los Angeles if she wanted to? Of course she could. Unquestionably. If you think Hillary Clinton can't do basic body mobilization when she needs to, you haven't been paying attention to American politics for the past few decades. There are unions in Los Angeles that can fill a stadium on a week's notice. This is not as big a task as it looks.
Now, I'll grant that this is probably easier for Sanders -- that he has more energy behind him. But let's loop back to the first question. Sanders turned out 104,000 people in seven counties in which 4.7 million people either voted for Obama in 2012 or are currently registered Democrats (depending on the metric available). Clinton can spot Sanders those 100,000 voters no sweat, even assuming that they're all Sanders backers, which is not a fair assumption. She's still beating Sanders nationally by a wide margin, and in every state primary poll, including Vermont-neighboring New Hampshire.
We know one thing about the Sanders campaign from these events: He carries with him a significant amount of grass-roots energy. Which we already knew.
If Sanders fills Cowboy Stadium or the home field for the Utah Utes, I'm happy to revisit the numbers. Until then, I'd recommend against extrapolating from 28,000 to 538.