The data in the chart above are appalling. They may also be misleadingly incomplete.
They show that the overwhelming majority of remaining self-identified Republicans support Trump. But they do not take into account whether self-identified Republicans are a declining part of the population.
Last night Missouri State Senate District 17 became the 46th political jurisdiction to flip from Trump-supporting Republican to Democratic in special elections held in 2017-18. In the same time period a whopping seven special elections saw a flip from Democratic to Republican.