In Chapter 9 of A People's History of the United States, why did denying blacks the right to vote contribute to the self preservation of northern men?

On page 203 in my edition of the book, Zinn quotes a
New York Times editorial that says that denying blacks the right to
vote was necessary "under the supreme law of self-preservation."  The idea behind this
was that giving blacks any sort of power would be dangerous to white
people.

This idea was based on the assumption that blacks
were dangerous to white people.  Right after the quote above, Zinn talks about the ways
in which whites saw blacks as animalistic.  Blacks were wild things who could be counted
on to attack whites whenever they were given the
opportunity.

If whites saw blacks in this way, their
reasons for denying blacks the right to vote are more understandable.  Whites felt that
blacks were inherently dangerous.  Giving them the right to vote would give them more of
a sense of power and make them more likely to harm whites.  Thus, denying them the right
to vote was an act of self-preservation.