Cowards can't block Warriors.
{R}: Target creature becomes a Coward until end of turn.
{2}{R}: Target creature becomes a Warrior until end of turn.
Note that the exact wording of this card has changed since it was printed in the _Future Sight_(TM) set, but its functionality is the same.
Coward is a new creature type.
Each of the activated abilities replaces all creature types the affected creature may have had.