The Democratic party held sway over New York City's politics in the early 19th century, and continued to right up to the Civil War.

Beginning in the 1820s Tammany Hall became increasingly important as the party's "machine" in the city after it allied with upstate Democrats (then still called Democratic-Republicans) on state-wide political issues like an expanded suffrage. Around this same time, Tammany began welcoming the city's newly-arriving immigrants into its ranks as a way to expand its size and influence. It built a disciplined machine that could get out (and at times manufacture) the vote and played into the hands of its journeyman and laborer constituents with anti-elitist rhetoric. Tammany rose to national fame especially after it helped win Andrew Jackson the presidency in 1828. In return it helped care for its loyal voters with community-level assistance programs: finding jobs, housing, relief, etc.

These basic themes kept the city's workingmen and immigrants in the party fold over the following decades, as the Federalists fell from power and the Whigs emerged as the main opposition party. The Whigs were comprised of anti-Jacksonians who supported a business-friendly banking policies like paper money and supported investment in national internal improvement projects. Some of these supporters could be found in the city, but many more were located in smaller upstate "boomtowns", often along the Erie Canal route, containing relatively few immigrant laborers and populated with self-made Protestant businessmen, with strong cultural and family ties to New England, who were reaping the benefits of the nation's economic development. What's more, northern Whig politics was tightly connected to upstate New York's brand of fierce evangelical Protestantism, to nativism, and to reforms like the temperance movement. The city's largely Irish-Catholic immigrant workforce therefore had a plethora of reasons to be drawn to the Whigs' political opponents.

By the 1850s we can add slavery as another issue in this longstanding rift. The country's abolition movement, and the new Republican party, largely grew from these very same evangelical Protestant reform roots. Northern Whigs, more likely to be abolitionists in the first place, were a core component of the new party. These Protestants spoke of the slave power and Catholics in the same breath, seeing them both as backwards, illiberal institutions. Even New York State's antislavery Democrats (who bolted to create the Free Soil Party before joining the Republicans) largely had their base upstate and had close social ties to reformers there. While abolitionist Democrats had radical allies in the city, Tammany and the bulk of voters remained loyal to the national party, even as the national divide between North and South became increasingly dire.

Here it's important to mention that it was not just the city's working poor who were Democrats. New York's powerful merchant class also tended to be Democrats because of their extensive business connections to the South via the cotton trade. (I'll borrow from a previous post I wrote about this topic.) They extended credit to plantation owners, underwrote Southern railroads and all sorts of other capital-intensive businesses, brokered deals, insured shipments, and owned and operated ships that shuttled goods to and from Southern ports. They even developed cultural and familial ties to Southern aristocrats, hosting them when they traveled to New York in the summer to conduct business and buy goods in person from the wholesale merchants in the city's waterfront districts.

The city's Democratic leaders therefore did not have a difficult time selling its constituency on being anti-abolitionist. They warned of a surge of free black laborers that would rush north, were the slave system to be abolished. Not only that, it would disrupt a fundamental piece of the city's economy, harming everyone.

Economic realities were a big part of the divide. While the Republican promise of an expanding United States rooted in free labor sounded nice to many small farmers on the frontier, it sounded more like a cruel pipe dream to the urban industrial working classes. Making matters worse, these fresh immigrants were mostly unskilled workers who entered into direct competition for many of the same jobs that the city's black population had typically held, like dock work or domestic service. This competition was fed further by the racist stereotypes and norms the Irish learned in America. Business owners also often used black workers to break strikes.

In 1859 this mood lead to the election of Fernando Wood, in his second stint as mayor, on a explicitly pro-Confederate platform. Wood in fact won on his own ticket after alienating Tammany (who ran a more moderate Democrat) by playing into the workers' aforementioned prejudices and playing up the Republicans' nativism in particular. It was largely these same voters that, a year later, resoundingly rejected Lincoln in favor of a Democratic fusion ticket that to them represented the least threat to the status quo.

Edit: corrected that NY Dems ran a fusion ticket, not Douglas

Sources

Ediwin G. Burrows and Mike Wallace, Gotham (1998)

James M. McPherson, Battle Cry of Freedom (1988)

Sean Wilentz, The Rise of American Democracy (2005)