LOS ANGELES — Was it something we said?

In the six months since President Trump took office, he has visited Iowa. Ohio. Florida. New York. South Carolina. Wisconsin. Georgia. Pennsylvania. Virginia. Michigan. Tennessee. Even New Jersey.

But the 39 million residents of the country’s most populous state have yet to receive a visit — a break in presidential travel history that dates back decades.

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Perhaps Mr. Trump is in no rush to visit the state that has emerged as the center of the resistance to his presidency. He lost California by more than four million votes in November. The state is the biggest Democratic stronghold in the nation, and its leaders have been challenging Mr. Trump and his policies at every turn. Mr. Trump even described California as “out of control.”

Or maybe it’s the president’s apparent aversion to long trips. Despite having the luxury of traveling on Air Force One — no taking off your shoes for a security line — Mr. Trump has stayed close to the East Coast since he took office, crossing the Mississippi River only once, briefly, for an Iowa rally last month.

And it can’t be for a lack of a place to lay his head. The Trump National Golf Club in Los Angeles promises to be the “pinnacle of the luxury golf experience,” good enough, presumably, for a president. (Except for his foreign trip in May, Mr. Trump has slept only at the White House, Camp David or a Trump property since becoming president.)

As he prepares to head even farther east on Wednesday for a four-day trip to Poland and Germany, the president has left some Californians wondering whether he has any intention to visit at all.

“Here’s a guy who talked all about how he was going to be the one Republican who can turn California — and he hasn’t been here once,” said Eric C. Bauman, the state Democratic leader. “I think considering that Trump lost California so big, he doesn’t feel he can get it back.”

White House officials say there is no deliberate effort to snub the state. Sarah Huckabee Sanders, the deputy press secretary, said they “just haven’t been there yet,” adding that “we aren’t basing the president’s travel on places others have been to, but on places where we are highlighting an issue, an industry.”

And Mr. Trump’s supporters, including some California Republicans, say that the president has little time to spare for the five-hour, cross-continental flight here, arguing that he is occupied in Washington with pressing issues like health care, tax reform and Syria.

“The president is very, very busy,” said Representative Dana Rohrabacher, an Orange County Republican. “I don’t think it’s strange at all. I don’t think any of us even noticed.”

Well, some people have noticed.

Since at least Jimmy Carter, every president has visited California by about the six-month mark in his presidency, looking for political support, campaign cash or perhaps just a few days of good weather.

In May 1977, Mr. Carter spoke in Fresno about the state’s drought. Ronald Reagan returned to his home state in June 1981. George Bush was in San Jose talking about taxes in April 1989. Bill Clinton, who had a well-known love for this state, visited Silicon Valley in February 1993, and George W. Bush was in Los Angeles in May 2001. Barack Obama visited California in March and October of 2009.

The list of states Mr. Trump has visited include ones he won last year, along with New York and Virginia, which have Trump-branded properties, and Connecticut, where he delivered the commencement address at the Coast Guard Academy.

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Steve Schmidt, a Republican operative who ran the 2006 re-election campaign of Arnold Schwarzenegger, the former Republican governor of California, said the country’s divisive politics may be partly responsible for Mr. Trump’s avoidance of the state.

“I suspect for President Trump it would be a lot like going to Canada, although much more hostile,” Mr. Schmidt said. “There are big swaths of the country which are no-go zones, which will spark protests and terrible coverage. I suspect you won’t see him going to those parts of the country.”

It seems fair to assume that Mr. Trump would not encounter in California the same kind of rapturous reception that he has drawn in other states. When Mr. Trump made his one trip back to New York, his home city, he confined the visit to the Far West Side of Manhattan, avoiding his Midtown home at Trump Tower, which has been the site of loud demonstrations.

“He is not going to get big crowds applauding him,” Mr. Bauman said of a Trump visit to California. “He is going to get big crowds booing him. He can’t stand being booed.”

Mr. Trump’s approval rating is even lower here than it is nationally: Just 27 percent of Californians said they approved of the job he was doing, according to a poll conducted in May by the Public Policy Institute of California.

“We don’t share the same values as a state,” said Kevin de León, the Democratic leader of the California Senate. But, he said, “if President Trump were to come to California, he would see we are in fact not out of control.”

Tom Del Beccaro, a former California Republican Party leader, said Mr. Trump had not come here — and probably should not invest the time in coming here — because the party was in so much trouble in Washington.

“For Trump to be in California would be like a luxury item,” he said. “It would mean that they are doing so well, they can afford to do those sort of things. But right now, they are behind in their agenda.”

Some Republican strategists say Mr. Trump might be missing an opportunity to be seen standing up to anti-Trump sentiment.

“You’d be well served to come to California and speak to his base back in Wisconsin and Michigan and Pennsylvania,” said Rob Stutzman, a California Republican strategist who works out of Sacramento.

“This is a president that loves foils,” Mr. Stutzman said. “California would be a great foil for him.”

If Mr. Trump changed his mind, a choice spot for a presidential visit would be Sunnylands, the sprawling estate in Rancho Mirage, built in 1966 by the publisher Walter H. Annenberg. It has been a favorite of Republicans in the past. Mr. Reagan often celebrated New Year’s Eve there and the elder Mr. Bush hosted the Japanese prime minister for a state dinner at the estate.

After his death, Mr. Annenberg’s foundation turned his winter home into a high-end conference center, complete with a golf course, for use by American presidents and other government officials to convene world leaders, marketing it as the Camp David of the West. Mr. Obama grew to like Sunnylands during his presidency, holding formal summit meetings there with President Xi Jinping of China and King Abdullah II of Jordan.

Accordingly, the foundation sent Mr. Trump a letter of invitation soon after his election, announcing that the estate was available for presidential business or pleasure.

The Annenberg Foundation is still waiting for an R.S.V.P.