The USA isn't anywhere near as anti catholic in OTL starting off. But Britain is.
1776- The declaration of independence is signed and the revolutionary war starts only this time Canada joins too. The colonies gain their independence and form the USA.
Things continue normally like OTL however tensions begin to rapidly grow as more immigrants from Europe come in and make the protestants feel that they may become a papist puppet. While the Catholics largely in Quebec feel that they are being displaced and losing their culture. On top of this the slavery debate rages.
1848- The mormon exodus still happens ITTL but during the Mexican American war the USA takes the sparsely populated north of Mexico.
1861- 1865 The American civil war happens and many people are fighting, it's brother vs brother. Then Robert E. Lee had a dream that he should surrender and convert to Mormonism. His word carrys so much weight that he pushed Mormonism into the mainstream in the South. Unfortunately, due to the nature of the South at the time many were scared of being overtaken by the freed slaves so they were quick to embrace polygamy to further displace the freed slaves. During this time more and more Catholics and protestants were immigrating to the USA.
1870- Shortly after the American civil war there was a massive uprising in Quebec to secede but this was crushed by the American Military. This led to Quebec losing many of the special privileges they received from the constitution during military occupation similar to reconstruction. French was also heavily discouraged in the state as well to try and Americanize them (think Louisiana). This led to mass riots from catholic populations all over America. The protestants saw this as a threat to their way of life and were quick to react.
1890- Just as things are looking like religion is going to tear the nation apart there is a wave of mormon missionaries as well as a nationalist movement. The mormons preached that the Catholics and protestants had been led astray. This leads to mass conversion among both protestants and Catholics and new immigrants struggle not to be absorbed into the new majority religion.
1898- This new nationalism would be put to the test with many groups formerly seen as Catholics (Irish, Italian, etc) Fight in the Spanish-American war and prove their loyalty to America.
1900 - Due to the incredibly high birthrates thanks to polygamy Cuba, Puerto rico, and even the phillipenes end up inter mixing and being over taken with large enough numbers of Americans to make them states. The phillipenes however was divided between 3 states. Hawaii, Guam, and Alaska are also states.
1917- America joins the great war and many people who were considered part of the protestant group (mostly germans) prove their loyalty to America in a similar fashion as the Catholics did in the Spanish American war.
Roaring 20s- America has unrivaled prosperity and unity as most people put aside their differences for the wellbeing of the nation. However one group lagged behind, the African Americans were largely ostracized by the mormons until the head pastor says that faith should know know color. Due to the man's word carrying such weight the nation rapidly desegregates and social change happens rapidly. The South is skeptical at first but buy the next generation they had bigger problems.
1930s- The great depression was a trying time however because of church influence things such as the new deal are never signed. Prohibition however is, prior to this there were several local laws banning alchohol but this was the first federal one. Surprisingly it worked too. The NFA also was never passed since without the negative effects of prohibition the mafias never rise to prominence.
1941- The US enters ww2 with a spark ignited. Historians would later say that it was like the Americans took every single piece of internal conflict they ever had in their history and unleashed the pent up anger on the axis.
1945- The war is over and the USA is stronger, United and powerful. But there is a new threat from the east that threatens their very way of life.
I'm gonna end it here. So what did yall think? This was my first real timeline so any and all criticism is welcome.