"People are fleeing California. Taxes too high, Crime too high, Brownouts too many, Lockdowns too severe," Mr Trump, 74, tweeted shortly after midnight.
"VOTE FOR TRUMP, WHAT THE HELL DO YOU HAVE TO LOSE!!!" he added.
California has been ravaged by multiple wildfires in recent months, some of which continue to rip through Golden State, leading some residents to flee.
The state was also hit with a record-breaking heatwave over the summer, causing multiple power cuts or "brownouts", as the president referred to them.
According to the latest polls, Mr Trump trails his Democratic challenger, Joe Biden, by some 30 percentage points in the state.
Democrats control every statewide office, dominate the legislature and hold all but eight of California's 53 US House seats. And the GOP’s deficit in voter registrations is a staggering 4.4 million.
With less than three weeks to go until election day, it remains to be seen whether the president's unorthodox pitch will have any impact in California.
People are fleeing California. Taxes too high, Crime too high, Brownouts too many, Lockdowns too severe. VOTE FOR TRUMP, WHAT THE HELL DO YOU HAVE TO LOSE!!!
— Donald J. Trump (@realDonaldTrump) October 15, 2020
She made the revelation in a lengthy note chronicling her personal experience with Covid-19, including being hit with a "roller coaster" of symptoms that she treated with vitamins and healthy food.
After she and the president tested positive earlier this month, the White House said 14-year-old Barron had tested negative. Barron later tested positive for the virus but had no symptoms, Ms Trump said Wednesday, adding that he has since tested negative again.
Mr Trump, speaking at a campaign rally Wednesday night in Iowa, was cavalier about Barron's infection, saying, "he had it for such a short period of time, I don't even think he knew that he had it."
"Barron is just fine," he added, using his son's quick recovery as part of his pitch to reopen schools. "It happens. People have it and it goes. Get the kids back to school. We've got to get the kids back to school."