There's a lot of debate on that, if you want to get into the academic side. I think the best definition for it (when I was studying it in college) was based on four primary 'focuses' of evangelism. The wikipedia article on the scholar that laid them out explains them in brief: https://en.wikipedia.org/wiki/David_W._Bebbington
I've seen them cheekily laid out with the '-centrism' part in every one, as in 'biblicentric,' 'crucicentric,' 'conversioncentric,' and 'activity-centric.' The wikipedia page on evangelicalism itself is a pretty big slog, but talks more in depth about them.

What those four things mean in practice is very interesting, again in an academic manner, in how neatly they lay the groundwork for MAGA/this shit we're seeing. The biblicentric primes them to reject the authority of more 'traditional' sects when there exists conflict between beliefs (even between different evangelical churches), because the bible is 'always right' and you can always find something in the bible to contradict something someone is saying, and is a major part of why evangelicals that move will church shop for a while (so they can find one that fits with their beliefs, highlighting, of course, that their beliefs are not necessarily based on much more than vibes). Crucicentrism isn't as big, but it's a part of their 'us vs. them' mentality that pops up. The emphasis on conversions and activism is part and parcel of why so many of them want to force their beliefs on others (think abortion bans and such).

Now take those religious aspects, and throw them at politics (where politicians specifically target them). They're already primed to reject logic, even christian logic, so republican (well, fox news style propaganda even more so) calls to reject what the other side says finds fertile ground. Use their desire to spread their beliefs in controlling, political manners, and you've roped in every one of them that wants to 'do christianity' with all the guilt they have built in about not improving anything suddenly being assuaged.