The data aren't in for 2024 yet, but swing voters were probably determinative in
previous cycles.
In 2020, there were even fewer swing voters than in 2016. However, it appears that swing voters favored Joe Biden over Donald Trump by about a two-to-one margin. Moreover, exit poll data confirm that swing voters tilted toward Biden in several key swing states. In Georgia, 6% of 2016 Trump voters switched to Biden while only 3% of 2016 Clinton voters switched to Trump. In Pennsylvania, 7% of 2016 Trump voters switched to Biden while only 4% of 2016 Clinton voters switched to Trump. Finally, in both Michigan and Wisconsin, 6% of 2016 Trump voters switched to Biden while only 4% of 2016 Clinton voters switched to Trump.[2] These differences seem small, but given the closeness of the 2020 presidential election in all of these states, they might have been large enough to shift their electoral votes from Trump to Biden and that would have been enough to change the outcome of the election in the Electoral College.
Democrat Party strategists cannot afford to write off swing voters who've ever voted for Trump as stupid and/or uneducable, as has been suggested upthread. Those sentiments may feel good when licking your wounds from an electoral drubbing, but giving in to that sort of thinking will make the next drubbing even worse.