Did the runner give the fielder every chance to field the ball? (No. The runner stays in the 'baseline' never varying his basepath, even with plenty of opportunity to go around.)
Back when, the rules said the runner had to go so far as to run behind any fielder with a chance at the ball. That rule is changed but the spirit remains: Did the runner deny ANY chance the fielder had to field the ball? Note that it is not necessary for the fielder to contact or be contacted by the runner.
Did the fielder do ANYTHING different than he would have were there no runner from first? Many of you are keen to point out that immediately after the fielder loses his opportunity to field the ball, he becomes liable for obstruction. You should apply the interference rule with the same zeal: until the time when the fielder no longer has any chance at the ball, the batter is liable for any action, including his presence, which hinders a fielder on defense. You don't need a train wreck or the runner blatantly getting hit by a batted ball.
This is interference. The fielder probably never realized that he has right-of-way, and likely meant to obstruct the runner from 1b in the split-seconds after he flubbed the ball. It doesn't matter since the ball is immediately dead- indeed, before the ball passes the fielder; even possibly before it is entirely past the runner since this is where the fielder is caused to hesitate.
The base umpire saw this differently, but no doubt he was late to the party. On a ground ball up the middle when there is a runner and a fielder headed for the same space, you need to spin around ahead of the ball so you can be stopped, with your eyes set, before this imminent collision.
Then when your partner bangs his belt, it will be because you were in position, not because you were lucky enough to not have a circus.
Nice synopsis from BalkHawk. Thank you for your contribution!