As a private pilot, I fly under Part 91. However, Wellstone had chartered that flight, so it was probably under the tighter restrictions of Part 135. Here's an example of the tougher restrictions:

Under Part 91, an instrument rated pilot can make an approach to a runway that is below required ceiling or visibility minimums, kind of a look-see to see if landing is possible. You're not allowed to descend below the minimum altitude for the approach unless you have the runway in sight, but some people are a little "optimistic" about what that means. Under Part 135, a pilot isn't allowed to attempt an approach if the reported conditions are below minimums.

Furthermore, while the King Air A100 is a good plane (it has been in production for almost 40 years), insurance companies have pretty tough pilot requirements. The plane is expensive to buy and operate, so owners are pretty strict about who they allow to fly the planes.

As you mentioned last week, one problem for many politicians, especially those running for statewide office, is that they have a lot of territory to cover and not a lot of time or money. Many of them accept rides from private pilot volunteers to get around, especially into smaller airports that aren't accessible any other way than by light planes. However, the pilots and their planes may not be up to the task of flying into small, unfamiliary airports at night or under instrument conditions. Working on my instrument rating, I learned one little tidbit that's sobering: IFR flight is more dangerous than VFR flight, and night IFR flight is about 3 times as dangerous as daytime IFR flight.

Complicating the issue, most light planes aren't as well equipped or maintained as larger planes like the King Air. Most light planes aren't equipped for flight into known icing conditions. Most light twin-engined planes have very marginal performance if one of the engines fail, too.