Is it possible for a major-party candidate to be both too liberal and too conservative? That was the quandary of the Washington Post in an article headlined, "Kaine Not Liberal Enough? Just Ask Virginians." The Post points out the irony between the criticism among groups claiming to speak for progressives ("a loyal servant of the oligarchy") and his record in public service ("near-perfect scores from an array of liberal interest groups").
A lot of people are learning about Kaine for the first time, or at least looking at a mass of information about him to develop an opinion. The rub is that, as I've written about before, our opinions tend to drive how we process new information rather than new information driving our opinions. This process, where, as Simon & Garfunkel put it so beautifully, "a man sees what he wants to see and disregards the rest" is known as motivated reasoning. The way we think about new information is driven by our existing belief system and frame, in this case whether you are to the right or left, resulting in Kaine being criticized for being both too liberal and not liberal enough all at once.
In the case of Bernie Sanders enthusiasts from the left wing of the party reluctant to support Hillary Clinton, they are disappointed in the Kaine choice because of his positions on, for example, trade and banking. To quote the coordinator of a group calling itself the Bernie Delegates Network, "If Clinton has reached out to Bernie supporters, it appears that she has done so to stick triangulating thumbs in their eyes."
For people starting with that belief system, they don't see an advocate of smoking bans in Big Tobacco's backyard or gun control in the home of the NRA. They certainly don't see someone who is capable of being criticized for being too liberal for Virginia. Bob Holsworth, a political analyst quoted by the Post, said, "Throughout his time in politics here, there has always been this question about whether Tim Kaine was too liberal for Virginia. No one has ever suggested this was a moderate who couldn't be counted on to support liberal values."
The partisan critics on both sides are like fans at a sporting event, viewing the calls on the field through the lens of whether the officials are trying to screw their side.
In fact, one of the first and most influential studies about motivated reasoning was conducted among football fans. In "They Saw a Game," published in 1954 by Professors Hastorf and Cantril at Dartmouth and Princeton, the professors showed their students a movie of a famously rough, violent Dartmouth-Princeton football game: a broken nose, a broken leg, another leg injury, a player kicked in the ribs while he was on the ground, and numerous infractions (called and uncalled) by both sides throughout the game.
From surveys of students watching the movie, you would have thought they were watching completely different games: the number and seriousness of each team's penalties and the blame for the violence differed radically depending on which team's fans were asked.
Sounds a lot like the reactions to the Kaine pick.