"Half the country" is what Trump supporters claim the size of their membership is. It's propaganda based on a purposeful misrepresentation of the fact that about about half of the public who voted in 2016 voted for Trump. They like to use it when making veiled neo-revolutionary threats against "the left" or "city dwellers" or whatever group they feel doesn't show them enough respect at that moment, or to overstate the legitimacy and influence of their movement within the public at large.

At some point, "half the country" simply evolved within common parlance into shorthand for Trumpists and Republicans in general. An unfortunate side effect of this is that it's also useful among the left, who should know better, when used unironically to make Trumpism into a bigger monster than it is.

Of course, nowhere near half the country even voted at all, and of those who voted, a slightly bigger half voted for Hillary Clinton. But Trumpism demonstrates that minority movements can still have influence when they grab power, even if they can't really claim a mandate. That a significant fraction of Republicans either believe the election was stolen or believe claiming such is necessary to maintain power is still troubling.