When you say populist approach the thing that comes to mind is the Vigilance committee that arose during the 1850's and 1860's in San Francisco. But that wasn't really populism it was the "best citizens" organizing lynch mobs to persecute unwanted racial groups and bringing law and order to the streets. When the Governor of California wanted to declare the city in state of revolt the US president didn't even want to send troops. The Vigilance committee eventually just took over city hall and created a legitimate political party.
Here's an attempt:
POD 1856-1858: Free-soilers in Kansas are far more successful in the conflict over slavery, they expel slave holders from the state and slaves are freed. In response southern states feel the Federal government even though it is sympathetic to slavery is unable to protect them. After a resoundingly ant-slavery constitution is passed in kansas, the southern states respond with raising their militias and declaring a secession an open. Somewhere a "Fort Sumter" like event occurs in early 1858 that leads to Civil War.
mini-POD: To help out not-yet Emperor Norton, his shipment of Peruvian rice comes through and instead of economic ruin he successfully invests in real estate in San Francisco and starts to eye political ambitions.
With the outbreak of the Civil War Governor Weller follows through with his promise to declare California an independent Republic in 1859.
The Vigilance Committee in San Francisco, seeing chaos and subversion everywhere is not a fan of Weller. With the charismatic speeches of Norton on their side, the new republic of California is already facing a serious problem. First the President Weller declares San Francisco in a state of insurrection, and then the Vigilance committee declares Weller in insurrection. Norton leads a successful sneak attack against Sacramento and declares himself Emperor Norton I of California, and the Popular Party wins elections with state wide support.
Norton may have come to power with the racially tinged politics of the Vigilance committee I could see him rolling back the excesses of the committee and creating a farely liberal regime.
Sources:
https://en.wikipedia.org/wiki/San_Francisco_Committee_of_Vigilance
http://governors.library.ca.gov/05-Weller.html