Republicans rallied the anti-slavery forces of the North and won.

Slavery not only racked the parties and caused new alignments; it
racked and split the Union. It is one of the remarkable phenomena
of our political history that the Civil War did not destroy the
Democratic party, though the Southern chieftains of that party
utterly lost their cause. The reason is that the party never was
as purely a Southern as the Republican was a Northern party.
Moreover, the arrogance and blunders of the Republican leaders
during the days of Reconstruction helped to keep it alive. A
baneful political heritage has been handed down to us from the
Civil War--the solid South. It overturns the national balance of
parties, perpetuates a pernicious sectionalism, and deprives the
South of that bipartizan rivalry which keeps open the currents of
political life.

Since the Civil War the struggle between the two dominant parties
has been largely a struggle between the Ins and the Outs. The
issues that have divided them have been more apparent than real.
The tariff, the civil service, the trusts, and the long list of
other "issues" do not denote fundamental differences, but only
variations of degree. Never in any election during this long
interval has there been definitely at stake a great national
principle, save for the currency issue of 1896 and the colonial
question following the War with Spain. The revolt of the
Progressives in 1912 had a character of its own; but neither of
the old parties squarely joined issue with the Progressives in
the contest which followed. The presidential campaign of 1916
afforded an opportunity to place on trial before the people a
great cause, for there undoubtedly existed then in the country