This question is in respect to the Champion's increased critical hit range.
On a roll of 18 or 19 (when this class feature becomes available), is the attack automatically a hit as it would be with a 20?
Yes for the Champion's instance of an expanded critical range.
Improved Critical specifically says you score a critical hit on a 19 or 20. A critical hit is a type of hit; by scoring one, you've also scored a hit. If the word 'critical' wasn't there, the ability would certainly read as if you couldn't miss on a 19 or 20.
This is in contrast to if the ability said something along the lines of 'a hit with a roll of 19 on the d20 is a critical hit'.
Only a roll of 20 is an automatic hit, unless a feature says otherwise
is followed up by this amendment, and the clarification of the second tweet sequence:
I had a brain glitch and was thinking of a theoretical crit. range increase. Specifics: Improved Critical does score a hit on a 19.
If a feature says you scored a critical hit, you hit.
In #dnd5e, a critical HIT is a hit. The fighter's Improved Critical doesn't say you CAN score one. It says you SCORE it.