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.
As Written in the Player's Handbook:
Rolling 1 or 20
Sometimes fate blesses or curses a combatant, causing the novice to hit and the veteran to miss. If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. In addition, the attack is a critical hit, as explained later in this chapter. If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target’s AC.
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
The Problem I see with these descriptions is they separate these two descriptions, 1 is a proper subset of the other (20's being Critical hits) but not the other way around, meaning a 20 automatically hits no matter the modifiers, but that doesn't necessarily mean having a critical hit, Automatically hits, no matter the modifiers. It seems odd to me why they would separate the wording. It is almost as they wanted to have a condition(s) that were critical hits but not automatic hits, but they always wanted automatic hits to be critical hits. Reminds me of ALMOST everything that makes you incapacitated, where you can't take action or reaction (but you can still technically move) BUT most things that incapacitate you also clearly state you can't move or movement is 0 (Symbol of Pain being one of the few examples I can think of that does not follow this logic). Again, it seems pedantic, however given DND's history with criticals (don't get me started on 3rd/3.5 "Confirmning" Crit mechanics) it does seem like this could have been the motivation behind why they are separate Rules for these.
It doesn't help that it never mentions being able to "Automatically hit" under critical hit description (aware that the word hit, does imply "hitting", but I'm not buying that as it is the name of a feature and it has always been named a critical hit as far back as 2nd edition, I am also aware every instance discussing things related to critical hits, appears to imply that you hit, despite the very separate description mentioned above.)
I am also aware that they made an errata that the wording is "This is called a critical hit" instead of "In addition, the attack is a critical hit." the problem again is the "If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the Targets AC" which seems to be a separate aspect of "Rolling 1 or 20" than, it is also a critical hit. This only ever comes up I know with expanded critical range (Champion fighters, hexblade, some paladins capstone, etc...) but I feel it is purposefully ambiguous to a fault.
If Jeremy Crawford wanted to clear it up, he could have simply said "All critical hits are Automatic hits, and All Automatic Hits are Critical hits" And that would clear things up, but again his answer is more akin to "It says what it says, and it is clearly saying it" which if people are asking for explanations, then it isn't :P, but then this would mess with things like the "Marut" creature that automatically hits, but they are not Critical hits... again implying that one is not a proper subset of the other. it's only one way conditional. it is not an If and only If...