TAMPA, Fla. — Hillary Clinton has vastly outspent Donald J. Trump on TV ads in Florida. Her 57 campaign offices dwarf Mr. Trump’s afterthought of a ground game. And Mr. Trump is deeply unpopular among Hispanics, who account for nearly one in five Florida voters.

Despite these advantages, Mrs. Clinton is struggling in the Sunshine State, unable to assemble the coalition that gave Barack Obama two victories here, and offering Mr. Trump a broad opening in a road to the White House that not long ago seemed closed to him. Mr. Trump is pressing down hard to win the state, campaigning in Miami on Friday and in Fort Myers on Monday, after a rally in Pensacola recently.

Recent polls show Mrs. Clinton is not earning the same support among Hispanics, young people or white voters that Mr. Obama captured when he eked out a Florida victory four years ago in the country’s most hard-fought swing state.

Mrs. Clinton could afford to lose here and still find other routes to victory. Mr. Trump’s electoral map is narrower; he must have Florida in his column. But as the most populous and one of the most racially diverse battleground states, Florida is also a bellwether for the nation: a candidate’s struggles here often are mirrored elsewhere.