Nothing!
That's one reason the 1st base coach is there. He/she has the option of moving in the coach's box, while the runner is "tied" to the base, in a manner of speaking.
If I were the runner, and the defensive player is standing in the baseline: once the ball leaves the pitcher's hand, I'm running into the defensive player (not in a malicious manner, of course) and asking for an obstruction call on my attempted steal.
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Lloyd
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