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.
The combat rules state that a critical hit is an attack that "hits regardless of any modifiers or the target's AC":
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, which is explained later in this section.
If you have a feature that allows you to score a critical hit on a 19, an 18, or a 2, then that attack will hit regardless of any modifiers or the target's AC.
That said, there are actually 2 caveats which are often overlooked that need to be addressed:
Critical hits only hit regardless of modifiers or the target's AC, but there are other things that can cause an attack to miss.
For example, if the target is hiding and the character must guess their location, if they guess wrong then their attack will miss automatically even if they roll a nat 20. This is not a modifier or AC, so even a critical hit can miss.
The only reason that this question exists is because "nat 20" has been conflated with "critical hit". These are two separate things, but it just so happens that by default if you roll a 20 then you will score a critical hit.
With Improved Critical you will score a critical hit on a 19 or 20. It is possible that in the future some effect will remove your ability to score a critical hit on a 20.