The GOP has been a very fragile national party for a while now, hence why they rarely win the popular vote. They are basically bolstered by the fact that the South is racially polarized so they had been winning a lot of populous states (GA, VA, NC, FL, etc. and now TX) by very small margins. The problem is that if the demographics change enough then they are on the other side of it (VA, soon TX). If Texas, Georgia and Arizona flip and start giving Democrats tiny margins, then there aren't enough swing states that make up for that.
I wouldn’t say GA , NC and VA pre 2008 were small margins
They were all pretty close in the Clinton era, and weren’t exactly Bush 43 landslides, although not particularly close either. Basically in the early 2000s the GOP hit a sweet spot in these states where the rural areas had already realigned to the Republicans but they were still strong in the suburbs. This coalition was only fleeting and was never going to hold long-term.
