Play: With a runner on 2nd and one out, the batter hits a soft liner over the shortstops’ head into short left field for a single. The runner on 2nd does not run, thinking that the ball might be caught. The batter is obstructed rounding 1st and continues running to 2nd, where the defense tags both runners. What is the correct ruling?
Ruling: The batter is out when tagged with the ball. The batter is entitled to the base that, in the umpire’s judgment, he would have reached safely had the obstruction not occurred. In the play you cite, the batter could not have reached 2nd base safely, because the runner at 2nd did not advance on the hit. Therefore, the batter is not entitled to 2nd base, even though he was obstructed while rounding 1st.
In cases when two runners are on the same base, the preceding runner has the right to occupy that base. If tagged with the ball by the defense before he returns to the previous base, the following runner is declared out.
Sources: See MLB Rules 7.03 (a, b) and 7.06 (a, b)