Donald J. Trump, confronting a daunting electoral map and a significant financial disadvantage, is preparing to fall back from an expansive national campaign and concentrate the bulk of his time and money on just three or four states that his campaign believes he must sweep in order to win the presidency.

Even as Mr. Trump has ticked up in national polls in recent weeks, senior Republicans say his path to the 270 Electoral College votes needed for election has remained narrow — and may have grown even more precarious. It now looks exceedingly difficult for him to assemble even the barest Electoral College majority without beating Hillary Clinton in a trifecta of the biggest swing states: Florida, Ohio and Pennsylvania.

President Obama won all three states in 2008 and 2012, and no Republican has won Pennsylvania in nearly three decades.

With a divisive campaign message that has alienated many women and Hispanics, Mr. Trump appears to have pushed several traditional swing states out of his own reach. According to strategists on both sides of the race, polling indicates that Mrs. Clinton has a solid upper hand in Colorado and Virginia, the home state of Senator Tim Kaine, her running mate. Both states voted twice for George W. Bush, who assiduously courted Hispanic voters and suburban moderates.