As the traditional Labor Day kickoff of the fall presidential race approaches, Republican Donald Trump faces an increasingly narrow path to the White House.
A Trump victory over Democratic rival Hillary Clinton likely would require a sweep of a set of battleground states where he is competitive but trailing in recent opinion polls—Ohio, Florida, Pennsylvania and North Carolina—and both campaigns describe them as the heart of the race. Mrs. Clinton, by contrast, could win with just one of them, partly because Democrats start with a larger number of states that historically side with them.
Both campaigns have put their money and their time in those states, despite suggestions they might turn elsewhere. Mr. Trump has traveled to those states more than others and has paid advertising in only these four states. Of the 20 media markets that have received the greatest number of ad spots placed by either the Clinton campaign or its main super PAC, 16 are in one of them, according to data from an ad tracker not affiliated with either campaign.
“It just becomes very hard and difficult to understand how the Trump campaign gets to 270” Electoral College votes needed to win, said Russ Schriefer, a strategist for Republican Mitt Romney’s 2012 campaign. Compared with that election, “the map has shifted and is much more favorable to the Democrats.”
Mr. Trump still has about 11 weeks to Election Day, and races tend to tighten toward the end. There is also enough time for unexpected revelations to alter the campaign’s trajectory. But polls show Mrs. Clinton has momentum.
Both her campaign and Priorities USA, a super PAC backing her, have pulled ads from Colorado and Virginia. They were battlegrounds in the last two campaigns, having voted Republican as recently as 2004 and in most elections in the decades prior.
But demographic changes favor the Democrats, and polling shows Mrs. Clinton with solid leads in both states.
If Mrs. Clinton carries those states and wins Pennsylvania, she could withstand losses in Florida, Ohio, North Carolina and many other swing states—assuming she holds the traditionally Democratic states. She also likely would win with a victory in Ohio or Florida, even without the Keystone State, a sign of her strong position. But Pennsylvania, with its long history favoring Democrats, may offer her the easiest path forward.
“Pennsylvania is key to the entire race” as a must-win state for Mr. Trump, said Mitch Stewart, who was battleground states director for Mr. Obama’s 2012 re-election. For Mrs. Clinton, Pennsylvania likely would be the “tipping point’’—the state that puts a candidate over 270 electoral votes when states are listed in order of the winner’s most-likely victories, he said.
Florida and Ohio, big states with large shares of electoral votes, long have been presidential battlegrounds, but demographic changes have made North Carolina, long a Republican stronghold, more favorable to Democrats. Then-Sen. Barack Obama won the state in 2008, though not in 2012.
While Mr. Trump’s first round of television advertising is restricted to four states, the Clinton campaign is airing ads in those states as well as in New Hampshire, Iowa and Nevada, three other swing states. It also is on the air in Omaha, hoping to pick off one electoral vote available in Nebraska, which allocates its share in part by congressional district.
Those decisions show the campaigns’ true priorities, amid predictions from Mr. Trump that he would win a string of Democratic-leaning states such as Minnesota, which hasn’t backed a Republican for president since 1972.
For her part, Mrs. Clinton’s campaign has funneled money for field organizers to Georgia and Arizona, where polling, unlike in Minnesota, shows tight races. On Tuesday, the Clinton campaign was scheduled to open a campaign office in Utah, which last voted Democratic in 1964.
Democrats say they don’t expect to necessarily win states beyond the core battlegrounds. But they hope the moves will entice the Trump campaign to shift money from top-priority contests into these traditionally Republican states.
Mr. Trump’s campaign didn’t respond to a request for comment. A Republican strategist close to the Trump campaign suggested the GOP nominee might want to spend some of his time and money in these traditionally Republican states. He cited the volatile immigration issue as having an unpredictable impact on Arizona and said it might be necessary to work on voter mobilization in Georgia.
Retaining Arizona, Georgia and Utah is essential for Mr. Trump, but for Mrs. Clinton, victory there likely would be Electoral College gravy. So, the Clinton campaign is focused on winning the states most likely to put her over the top, starting with Pennsylvania, Florida, Ohio and North Carolina, said Marlon Marshall, Mrs. Clinton’s director of state campaigns and political engagement.
“Those are states that will be crucial to getting us to 270 electoral votes,” he said. “Our goal is what’s the most efficient way to get to 270, and how do we open up as many pathways as possible.”
To deny Mrs. Clinton a victory in Pennsylvania, Mr. Trump is courting white, working-class voters outside the big cities. Democrats are relying on suburban voters, particularly women who might otherwise vote Republican. The Real Clear Politics average of public polls has Mrs. Clinton with a nine percentage point lead in the state.
Heading into the 2016 election, Democrats enjoyed a built-in advantage in the Electoral College. In every election since 1992, Democrats have won a set of 18 states, plus the District of Columbia, that add up to 242 Electoral College votes. Republicans have consistently held 13 states with 102 electoral votes.
Mr. Schriefer, the GOP strategist, said that given current polling, the challenge for Mr. Trump is to increase his popularity with broad swaths of the electorate, not to turn things around in any particular state.
He recalled many past GOP nominees, including Mr. Romney, who made late-in-the-game plays for Pennsylvania, only to see Democrats hold the state. “Pennsylvania for Republicans is always a little bit like Charlie Brown and the football. It gets pulled away from us at the last minute,” he said.
The Republican strategist close to the Trump campaign said the GOP nominee has some additional options, predicting that his message would resonate in places where there is economic anxiety, such as the traditionally Democratic states of Michigan and Wisconsin. Victories there would give Mr. Trump more options for assembling states for an Electoral College majority, but public polling shows Mrs. Clinton well ahead in both places. The Clinton campaign has sent field organizers to both states but so far hasn’t spent any money on TV ads for either place.