Was a bloody civil war the only way to end slavery in America by the middle of the 19th century?

This is, of course, a matter of opinion.  I would argue
that a civil war was the only way to end slavery quickly by
the middle of the 19th century.

It seems entirely possible
that slavery would have eventually been done away with by peaceful means.  We know that
Brazil abolished slavery in the 1880s without having to go through a civil war even
though slavery was a very important part of their economy.  There are a number of
reasons slavery might have ended in the US.

First, there
are economic reasons.  Slavery flourished before mechanization.  As agriculture became
more mechanized, mechanized farms would have outcompeted slave plantations.  This would
have made slavery much less economically viable.

Second,
there are moral/political reasons.  If the US had remained the only country with a slave
system (after Brazil abolished theirs) a great deal of pressure would eventually have
been brought to bear on it.  Other countries might eventually have gone so far as to
refuse to trade with the US if slavery still existed.

For
economic reasons or for political reasons, slavery would have ended at some point. 
However, it would have taken a long time for this to happen.  A war was the only way
that slavery could have been ended in a timely fashion.