The 100-day sprint to the US election began Sunday, the final act in a campaign transformed by an assassination attempt and the stunning exit of President Joe Biden. After weeks of infighting and despondency over Biden’s candidacy, Democrats have largely consolidated behind Vice President Kamala Harris, radically reshaping a race that was fast becoming Republican nominee Donald Trump’s to lose. Republican strategist Matt Terrill said Harris’s uniting of the Democrats had ensured a photo finish on November 5 — a ballot that will largely be decided by around 100,000 swing voters in a handful of battleground states. “It comes down to those independent, undecided voters. Inflation, immigration, the economy and crime — those are the issues they care about,” he told BBC News. “Right now, I think, former president Trump is doing quite well on those issues. This election will be a referendum on the incumbents in office — that’s still Biden and Harris. We’ll see how that takes shape,” Terrill said. While American election campaigns typically last almost two years, the 2024 edition has effectively been reset, making it unofficially the shortest in modern history. The Democratic convention in mid-August is expected to be a jubilant celebration of the party’s new standard-bearer Harris, who is enjoying record fundraising, growing grassroots support and an early boost in polling. It all looked so different just a month ago. Dogged by voters’ concerns about his age and mental acuity, the 81-year-old Biden was an outside bet at best, trailing his predecessor in the first presidential rematch since Dwight Eisenhower trounced Adlai Stevenson in 1956. Biden’s dismal June 27 debate showing ignited a five-alarm fire within his party. The flames were fanned by a flawless show of unity behind 78-year-old Trump at the Republican national convention — an event galvanized by the failed bid, just days earlier, to assassinate the former president at a rally in Pennsylvania. After an initial show of defiance, Biden bowed to the inevitable and dropped out last weekend. Harris, a generation younger at 59, threw her hat in the ring — turning what had been a stale contest between two unpopular, aging, white male candidates into a dynamic and unpredictable showdown. The former Biden-Harris ticket — now just the Harris campaign — held its biggest-ever rally Tuesday in Wisconsin and has raised more than $120 million in recent days, with disenchanted donors returning to the fold. Trump’s previous three-point nationwide lead in polling averages has been practically halved in a week, and the contest has become a margin-of-error tussle in most of the crucial swing states that decide elections. But Democrats enjoying the sugar high of the last week have been cautioned by party elders to sober up, with Harris still facing an uphill battle to beat the oldest major party nominee in history. “Before long, Harris’s ‘honeymoon’ will end and voters will refocus on her role as Biden’s partner and co-pilot,” Trump pollster Tony Fabrizio wrote in a memo this week. Democratic strategist James Carville told MSNBC that Democrats needed to cut the happy talk and prepare for the coming storm. “They’re coming at us and they’re going to keep coming. And this kind of giddy elation is not going to be very helpful much longer because that’s now what we’re going to be faced with,” he said. Even former president Barack Obama has cautioned against hubris, acknowledging as he endorsed Harris that Democrats are “going to be underdogs” and that she would have to earn the trust of voters.Trump, who has seen his favorability ratings tick upward since the July 13 attempt on his life and the successful Republican convention, will rally this weekend in the traditionally Democratic state of Minnesota. Harris, meanwhile, heads to Massachusetts for a fundraising event and will send her surrogates — including some favorites in the contest to be her running mate — out across battleground states.