The Democratic Party totally dominated elections in the American South from 1890 until after mid-20th Century. The reason for that was the bitter residue left by the Civil War and Reconstruction. Republicans had great difficulty electing anyone to a major political office in the South. White Southern Republicans were few in number, and the black vote was suppressed. Political primaries therefore became the important contests, and in these primaries the Democrats usually split between conservative and liberal factions. Democratic dominance was so great that the general election was usually no contest.
On the national stage the Republicans, the Grand Old Party that had preserved the Union, had a slight numerical advantage. It was the party of business, commerce, and prosperity. The Democrats appealed to the less privileged and to the immigrants, and the Democrats had the solid South.
Republican dominance in the North usually led to victory, but the Federal elections were always competitive.
For decades after the Civil War Republicans were considered the friend of the blacks, but that slowly began to change during the Depression, the presidency of Franklin Delano Roosevelt, and the beginning of massive public assistance programs. As a result of the civil rights struggles of the 1960s-70s, the political spectrum shifted even further. National Democratic leaders like Kennedy and Johnson took actions favorable to blacks, and Johnson aggressively advanced civil rights legislation. Republicans joined in the fight for racial equality but opposed many elements of Johnson’s “Great Society” program. Social turmoil resulting from the civil rights campaign caused Southern Democrats to become disaffected with Democratic Party leadership, and in the late 60s and early 70s large numbers of them shifted their political allegiance. Those formerly known as conservative Southern Democrats became Republicans, while liberal Southern Democrats remained with the Democratic party. Despite their former ties to the Republican party (After all, Lincoln had been the great emancipator), the now politically active blacks flocked to the Democratic standard. Many Southern states thus became politically competitive, with white liberal Southern Democrats and blacks facing off against former conservative Democratic (now Republican) candidates.
The infusion of conservative Southerners into the Republican ranks made that party much less attractive to blacks. It also had other interesting results. Many of these Southerners had strong evangelical Protestant roots, and this influenced a heavier emphasis on traditional cultural issues by the Republican Party. At the same time, the Democratic Party was becoming more secular. For the Democrats the main social concerns were promoting a fairer distribution of wealth, boosting labor unions, supporting universal health care, championing abortion rights and other women’s issues, defending the LGBT community, backing liberal immigration policies, etc. The Republicans also fought for social issues, but their emphasis was on more conventional lines such as defending time-honored family values, limiting the size and power of the Federal government, reducing taxes, promoting business, insuring domestic order, protecting property rights, and upholding the sanctity of life (including the unborn). The Republicans also prioritized the maintenance of a strong national defense and the protection of our borders.
Since the Presidential election of 2016, many evangelicals and conservative Catholics have been disturbed by Trump’s often egregious conduct, but they appear to be more afraid of left-wing Democratic extremists than they are frightened or offended by Trump. Opposition to Hilary Clinton, Nancy Pelosi, Chuck Schumer and Democratic socialists has created some strange bedfellows in the Republican Party.
Over the longer run, alterations in the character of the Democratic and Republican parties appear to be having interesting results. The increasing secularization of America would seem to favor the much more secular Democratic party, but there are side-currents. Many traditional Catholics are repelled by the Democrat’s increasingly secular-humanistic approach to governance. Though long-time Democratic supporters, some of these Catholics are beginning to change their party affiliation. The same is true of church-going Southern blacks. They are much more akin in their political/social values and attitudes to Republicans than they are to many of the prominent Democratic liberals who now represent their party, and a few of them are beginning to waver. Lastly, the newly arriving immigrants from Central and South America are naturally drawn to the Democratic Party, but over the long term many of them may also come to realize that their social values do not match those of the Democratic leadership.
What does the future hold? Only time will tell.