Democrats quickly rallied around Vice President Kamala Harris as their likely presidential nominee Sunday after President Joe Biden's decision to bow out of the 2024 race.However, the fast-moving political situation remained volatile just months before the November election.Shortly after Biden stepped aside, he firmly endorsed Harris, who would make history as the nation's first Black and South Asian woman to become a major party's presidential nominee. Other endorsements flowed from former President Bill Clinton and Hillary Clinton, the first major female presidential nominee, and prominent U.S. senators, a wide swath of House representatives and members of the influential Congressional Black Caucus.As Democrats rush to confront Republican Donald Trump this fall, Biden said choosing Harris as his vice president had been the "best decision" he ever made. "Let's do this," he said."My intention is to earn and win this nomination," Harris declared in a statement.Yet, the political and logistical situation ahead for Democrats is difficult and highly uncertain as the party that had expected to sail to November's election portraying Trump as a quasi-authoritarian leader and threat to American democracy, now must rearrange the top of its ticket in a matter of weeks — before the party's nominating convention begins Aug. 19.A groundswell appeared to be gathering for Harris to lead the party — within hours, Biden's campaign formally changed its name to Harris for President, reflecting that she is inheriting his political operation — but there are notable holdouts.Former President Barack Obama and Speaker Emerita Nancy Pelosi, who are widely seen as engineering Biden's withdrawal from the race as they worried about not only keeping the White House but winning control of Congress, did not explicitly endorse Harris, and Pelosi favors an open primary.Key Democrats believe Harris would benefit from a "mini-primary." They argue a fast primary campaign would showcase to the American people, party donors and skeptics Harris is best for the job, and give would-be contenders a chance to compete — or at least debut as potential running mates.Video above: Presidential historian reacts to Biden's decisionHarris was making calls late in the day to congressional lawmakers, including Rep. Annie Kuster of New Hampshire, the chair of the New Democrat Coalition, a moderate caucus on Capitol Hill. Kuster also endorsed Harris Sunday.Democratic National Committee chairman Jaime Harrison promised a "transparent and orderly process."With early voting underway in a matter of months, Democrats have no time to spare if they hope to compete against an energized GOP that has embraced Trump's return. They must quickly pivot to unify a shaken Democratic party, refocus an entire campaign apparatus around a new presidential ticket and organize the ground game to get out the vote.Trump's team faces its own daunting challenge, refocusing its relentless attacks, from Biden's age, stamina and ability to govern to a yet-to-be-named Democratic nominee, who could be Harris or any number of possible contenders, including critical swing state governors Roy Cooper of North Carolina and Josh Shapiro of Pennsylvania, as well as Sen. Mark Kelly of Arizona.But Republicans have also signaled their early strategy as criticizing Harris as complicit in standing by Biden, despite his fumbles, and they are working on legal challenges to the 2024 election."Not only would Harris be a disaster in the White House, but she also helped Biden cover up his declining health while in office, which destroys her credibility," said a statement from the Republican National Committee chairman Michael Whatley and co-chairman Lara Trump.Even with Biden's endorsement of Harris, there's been an active debate over her rise among Democratic heavyweights — lawmakers, deep-pocketed donors and former high-ranking officials of the Biden, Obama and Clinton administrations, according to a Democrat with deep ties to the Biden-Harris administration.The person, who spoke on the condition of anonymity to discuss internal conversations, said a mini-primary would help Harris seal her bona fides as a strong, sharp candidate and help diminish criticism that she's been undemocratically anointed—something that the Trump campaign has already sought to use against her.But for many Democratic lawmakers who immediately backed Harris for the nomination, it would be untenable for the party to skip past what would be her history-making candidacy.Washington Sen. Patty Murray said she is behind Harris "100 percent." Virginia Sen. Mark Warner said Harris "has the experience, energy, and resolve to lead our nation." Connecticut Sen. Chris Murphy said he will be "enthusiastically supporting my friend."Hawaii Sen. Mazie Hirono, Minnesota Sen. Tina Smith, Virginia Sen. Tim Kaine and California Sen. Laphonza Butler also said they would support Harris, as did a growing number of Democratic House lawmakers.Still, other Democrats have been silent on supporting Harris, or pushing for an open process.Democratic Sen. Peter Welch of Vermont said in an interview with The Associated Press that he thinks the party needs to "be making our decisions first and foremost about who can best represent us so we can defeat Trump."Donors are weighing in. Chad Griffin, a member of the campaign's national finance committee and a top Democratic fundraiser in the Los Angeles area, said the party is lucky to have Harris. "She's the trusted, tested leader we need to carry us to victory in November," he said in a statement.Video above: Sen. Ed Markey thinks Kamala Harris will bring 'X Factor' to presidential raceThe Democratic convention in Chicago, which was supposed to be Biden's coronation, now becomes an open contest in which nearly 4,700 delegates will be responsible for picking a new standard-bearer.The path ahead is neither easy nor obvious. Biden won every state primary and caucus earlier this year and only lost the territory of American Samoa. At least 3,896 delegates have been pledged to support him.Current party rules do not permit Biden to pass them to another candidate. Politically, though, his endorsement is likely to be very influential.The immediate burden is on Harris to solidify support across almost 4,000 delegates from the states, territories and District of Columbia, plus more than 700 so-called superdelegates who include party leaders, certain elected officials and former presidents and vice presidents.__ Associated Press writers Bill Barrow in Atlanta, and Mary Clare Jalonick, Aamer Madhani and Chris Megerian in Washington contributed to this story.