Contentious issues can be solved on a local level to prevent large scale unsolvable debates across a huge area. In the US Issues such as gay marriage, drug controls, gun control, and the death penalty are extremely divisive. States have gone in radical directions on all these issues. Allowing states to decide their own stance on these has prevented conflict on a national level. Many states within nations lean left or right, which can cause serious divisions in national elections. In the US, Liberals in California and New York do not have to have policy dictated to them by conservatives living in the South, and vice versa. Federalism can make these ideological divisions less of a fraught battleground because states are allowed to go their own way ideologically, creating their own laws to some extent. Federalism allows individuals to have their say on controversial issues on a local level. Devolving some powers to local administrations diffuses tensions caused by difficult topics.
The most controversial issues of all tend to be the ones that many people wish to see implemented uniformly across the nation. When the central government cannot accept the individual wishes of states and vice versa, conflicts form. One of the best examples of this is the American Civil War. Leading up to the American Civil War, America split into two opposing factions over slavery. The inability of the states and central government to come to an agreement was one of the factors which led to the Civil War.  In more recent times in the US, issue such as mask-wearing to prevent the spread of Covid-19, and abortion have created a huge struggle between states and the central government, with many people believing the central government most impose rules on individual states. The central governments often get into fights with their member states. In the US the conflict between different groups over how power should be devolved has been constant, stretching back to the founding fathers. More recently the New Federalist movement in the 20th Century, led by Ronald Regan, objected to Roosevelt's New Deal for undermining some state rights. Historically, federalism has not prevented conflict and has even caused it. Few can agree on what powers should be devolved to who and to what extent. Many people see a need to dictate major controversial or moral issues from the center.
[P1] Contentious issue can be decided on a local level to prevent conflict [P2] People can move to areas which reflect their views [C] Federalism helps democracy by diffusing conflict
Rejecting the premises