How Does the Delta of an ATM Option Change as Time to Expiration Decreases?

As time to expiration decreases, the Delta of an ATM option becomes more volatile and extreme. It will rapidly move towards either 1.0 (if the price moves slightly ITM) or 0.0 (if the price moves slightly OTM).

This is because with less time, the option has a smaller window to move back. This rapid change in Delta is reflected by the option's increasing Gamma.