Los Angeles (AFP) - A campaign for California to secede from the rest of the country over Donald Trump's election is gaining momentum, with supporters allowed to start collecting signatures for the measure to be put to a vote.

California's Secretary of State Alex Padilla gave the green light on Thursday for proponents of "California Nationhood" -- also known as Calexit -- to start collecting the nearly 600,000 signatures needed for the measure to qualify on the November 2018 ballot.

The 585,407 signatures required by July 25 represent eight percent of registered voters in California -- the most populous state in the country with nearly 40 million residents and the world's sixth-largest economy.

Should the initiative make it on the ballot, a "Yes" vote would repeal clauses in the California Constitution "stating California is an inseparable part of the United States and that the United States Constitution is the supreme law of the land," a statement by Padilla's office said.

Voters would then need to decide in another referendum in 2019 whether California should become a separate country.

Padilla said the independence measure -- deemed highly unrealistic -- would have a deep impact on the state and would likely face legal challenges.

"Assuming that California actually became an independent nation, the state and its local governments would experience major, but unknown, budgetary impacts," he warned.

"This measure also would result in tens of millions of dollars of one-time state and local election costs."

Calexit enthusiasts, whose campaign is called Yes California, are pushing for independence on grounds the state is out of step with the rest of the US and could flourish on its own.

"In our view, the United States of America represents so many things that conflict with Californian values, and our continued statehood means California will continue subsidizing the other states to our own detriment, and to the detriment of our children," according to their website.

The idea of independence became very appealing to many Californians following Trump's shock election.

Democratic candidate Hillary Clinton easily beat Trump in the progressive state, winning by more than 4.2 million votes, almost double the number of ballots cast for Trump.