Clinton's Pneumonia Jolts the Presidential Race

Hillary Clinton's campaign said Sunday she had been diagnosed with pneumonia and would cancel a planned two-day swing through California, hours after the Democratic presidential nominee abruptly left a 9/11 memorial ceremony in New York for what her aides described as her feeling "overheated."

The diagnosis, coupled with a remark by Mrs. Clinton late Friday criticizing some Trump supporters as a "basket of deplorables," is an unwelcome distraction for a campaign facing a tightening of polls in recent weeks.

Amateur video taken Sunday near Ground Zero in New York showed Mrs. Clinton looking wobbly as she got into her motorcade with an assist from staff and Secret Service agents. The 68-year-old went to her daughter's apartment and emerged about two hours later, waving at the waiting cameras.

"I'm feeling great," she said. "It's a beautiful day in New York."

Her Republican challenger, businessman Donald Trump, has sought to fan concerns about Mrs. Clinton's health, questioning her stamina and chiding her for keeping what he says is a light campaign schedule.

Mrs. Clinton's doctor examined the candidate at her home in Chappaqua, N.Y., later Sunday and said in a written statement that she had been diagnosed two days earlier with pneumonia. The doctor on Sunday said she had been dehydrated and overheated and was "recovering nicely."

The Clinton campaign didn't respond to a request for comment about why they didn't reveal the diagnosis earlier. Mr. Trump hadn't commented on the matter as of Sunday night.

The campaign said Mrs. Clinton was canceling a planned trip to California on Monday and Tuesday. She had planned to attend fundraisers and tape an appearance on Ellen DeGeneres's talk show.

Mr. Trump, who is 70, has called on Mrs. Clinton to release more detailed medical records. In December 2012, Mrs. Clinton fainted and suffered a concussion. She was hospitalized and treated for a blood clot in her head.

In July 2015, her personal physician wrote a letter saying Mrs. Clinton was in "excellent physical condition and fit to serve" as president. The letter said Mrs. Clinton suffered from hypothyroidism and seasonal pollen allergies.

With polls showing voters question Mrs. Clinton's honesty, the delay in revealing her condition after Sunday's incident could further damage her credibility, critics said.

"I can't understand the Clinton operation. You have to frankly tell people what happened and do so right away," said Ari Fleischer, White House press secretary under President George. W. Bush. "If you do that, these things really are not big deals. They only become big if it doesn't appear you're dealing straight."

Mr. Trump himself has come under attack for putting out few details about his health.

The Clinton campaign's recent difficulties stand in contrast to a summer in which its allies saw a position so commanding they didn't think Mrs. Clinton needed to do much in the way of campaigning. She spent parts of August holed up in private fundraising events.

Mr. Trump, meanwhile, was on the defensive over a back-and-forth with a Gold Star military family and over other comments. The Republican ousted top campaign advisers and hired new ones, and Mrs. Clinton largely kept out of the spotlight.

But the release in recent weeks of new documents on Mrs. Clinton's use of private email while secretary of state, as well as news coverage about Mrs. Clinton's ties to her family foundation's donors, have weighed on her poll numbers. An average of polls by Real Clear Politics shows Mr. Trump down by just 3 points.

Whit Ayres, a Republican pollster, said polls showing that so many voters view Mrs. Clinton as unlikable and untrustworthy are "all you've got to know to figure out why this race is reasonably close." Polls have shown Mr. Trump regarded even more unfavorably.

A fresh round of polls shows the race tightening in important battleground states. A pair of Democratic states in the last two presidential races—Nevada and New Hampshire—are now too close to call, according to a new Wall Street Journal/NBC News/Marist poll.

Another potential problem for the campaign was the remark Friday night. Assessing some of Mr. Trump's voters at a fundraiser, Mrs. Clinton said about half fall into what she called "the basket of deplorables."

The next day, Mrs. Clinton had put out a statement saying she was "wrong" to have demeaned some Trump voters in this fashion.

"There's no value in attacking a candidate's voters for how they vote," said Chris Kofinis, a Democratic strategist. A better strategy, he said, is to keep the focus squarely on one's opponent.

Mrs. Clinton still profits from an electoral map that gives Democrats an edge in the race for 270 electoral college votes. And the same Wall Street Journal/NBC News/Marist poll also offered troubling news for Mr. Trump. In two reliably Republican states, Mr. Trump led only narrowly: In Arizona, by one percentage point; in Georgia by three.

The Journal/NBC News/Marist results show how the traditional electoral map is being scrambled in a year that could see a realignment of both parties' coalitions.

Mr. Trump is making inroads among working-class, white men but alienating many Hispanic voters with his rhetoric about Mexico and illegal immigration. Mrs. Clinton is scoring gains among college-educated white voters, a bloc that Republicans have carried handily in the past.

"As we enter the final lap of this very unconventional election, it would not be surprising if the electoral map, in the end, has new contours," said Lee M. Miringoff, director of the Marist College Institute for Public Opinion. "Any of these four states [Nevada, New Hampshire, Arizona and Georgia] could awaken a fault line in what is looking more and more like a shake-up election, with more states being up for grabs."

These four states have historically been less competitive than vote-rich battlegrounds such as Ohio and Florida. President Barack Obama won New Hampshire and Nevada by more than 5 percentage points in 2012, while GOP nominee Mitt Romney won Georgia by nearly 8 points and Arizona by 9.

Now, the states look more competitive. Sensing opportunities in traditionally Republican territory, Mrs. Clinton is airing advertising in Georgia and Arizona.

The polls found that voting patterns in these four states are similar to trends in other places, with Mrs. Clinton leading among women and Mr. Trump winning among men.

Laura Meckler contributed to this article.

Write to Peter Nicholas at peter.nicholas@wsj.com and Janet Hook at janet.hook@wsj.com