Pg. 194 PHB:
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.
Pg. 72 PHB:
Beginning when you choose this archetype at 3rd Level, your weapon attacks score a critical hit on a roll of 19 or 20.
Since the PHB Defines a critical hit as a 20 that always hits regardless of modifiers or the target's AC, and improved critical states that a critical hit is scored (a miss cannot be called a hit) on a roll of 19-20, this means that a 19 will be an automatic hit because a critical hit ignores modifiers and the targets AC
Furthermore, on Pg. 196 PHB it states "When you score a critical hit, you get to roll extra dice for the attack's damage against the target."
Since you cannot roll damage if an attack does not hit and the improved critical feature explicitly states that you score a critical hit (the same wording used when describing the effects of a critical hit), this further confirms that a roll of 19-20 will be an automatic hit.
All emphasis was mine, I hope this helps!
Also feel free to ignore this in your game if you're the DM, it's up to you to use RAW or RAI!
Edit: didn't realize this was already confirmed by Jeremy, but at least you all have an in depth explanation for why
I don't see how the changed wording in this respect makes any difference to the interpretation of the Champion ability. This is called and in addition, this is do not seem to carry very different meanings.
Thanks for pointing that out! I didn't play 5th edition at that time so I've only ever had the copy that is written the way I quoted!
If earlier printing's text describing a critical hit on Pg. 196 is the same as the current edition then there would still be solid evidence to Improved critical being an automatic hit, but as I said, I never owned an earlier printing of the book!
I realize that this is a real corner case, but it seems like there is a separation of critical hits and automatic hits in the rules. The rules state that a 20 automatically hits, but it separately states that a critical hit does double damage. Where this gets interesting for me is a case where somebody has an expanded critical range and through high enemy AC and low bonuses rolls a 19 on die but does not meet the enemy's AC.
I think this (difficult to encounter) case is a miss. Does anyone have a different opinion? I entered D&D at the September play test. Have the rules always been this way on this subject in earlier editions?