He won't be able to withdraw from the treaty itself.

He'll be able to publicly say he won't defend NATO allies, he'll be able to withdraw troops, withdraw diplomats, withdraw ambassadors, no longer have US personell attend meetings, refuse to continue funding NATO HQ, sabotage command and control, undermine leadership, and on and on, until the NATO treaty is barely worth the paper it's written on, leaving European NATO wholly unprepared for a potential invasion. It's too late to prepare for that if they start right now.

Russia might then take a gamble. A lot of people thought they wouldn't take that gamble in 2014. People thought they wouldn't take that gamble in 2022. People think they won't take that gamble if Trump gets re-elected.

Or Russia doesn't take that gamble. They simply engage in provocations. Military exercises near the border. Bomber runs which are aborted at the last moment. Some more extravagent extra-territorial assassinations. The chance of a miscalculation skyrockets, the chance of accidentally starting a war increases significantly.

I suppose he could order the US military to physically leave NATO bases, and physically eject NATO allied personnel from American bases.