Don’t automatically assume the 30% who don’t bother voting are guaranteed to vote against Trump. Historically, it has been younger voters who don’t bother voting, and they do tend to be more liberal. But this time around, young voters are starting their adult lives in a world that is stacked against them, and they know it. They have felt the effects of post-pandemic inflation but have not had their pay raised to match. Those who were old enough to be adults pre-pandemic might very well conclude that they were better off personally when Trump was in office, and may be dismissing all the alarm at his re-election.
There have been several studies indicating that “disengaged” voters, if they show up, would break toward Republicans this time around. This article is from before Biden quit, but shows a clear preference for Trump among these voters: