DES MOINES, Iowa (AP) — After months of speculation and secrecy, the 2020 presidential primary season is about to explode.

With several Democrats already in the race, a half dozen more are locking down final travel, staffing and strategy to launch White House bids in the coming weeks. While plans may change, the announcements are expected to come in waves, the first featuring a group of ambitious Senate Democrats including New York Sen. Kirsten Gillibrand, California Sen. Kamala Harris and New Jersey Sen. Cory Booker, who face pressure to join the race after Massachusetts Sen. Elizabeth Warren’s entrance two weeks ago.

The second wave will likely feature political heavyweights like former Vice President Joe Biden, Vermont Sen. Bernie Sanders, New York billionaire Michael Bloomberg and former Texas Congressman Beto O’Rourke, whose advisers believe they have sufficient financial backing and name recognition to join the crowded field on their terms later in the first quarter should they decide to run.

“It is really starting to heat up,” said Deidre DeJear, who lost her bid for Iowa secretary of state last fall, but remains an influential figure in the state’s first-in-the-nation Democratic primary. She was among a group of Iowa Democratic women who sat down with Warren last week in suburban Des Moines.

“I feel like Warren put people in a place as if to say ‘come on, step up,’” DeJear said in a subsequent interview. “If you’re in it, you’re in it. No reason to wait now.”

Interviews with senior aides for several top Democratic prospects, who spoke on the condition of anonymity to discuss internal planning, indicate the waiting game is almost over.

Former Colorado Gov. John Hickenlooper, who left office last week, added foreign policy adviser Jeremy Rosner, finance director Dan Sorenson and a senior communications adviser Marie Logsden to his political action committee in recent days, according to an aide. Hickenlooper is traveling to Los Angeles this week to meet with donors and has scheduled visits to early voting states later in the month.

The 66-year-old term-limited governor and former businessman, who remains largely unknown to many primary voters, does not plan to launch his campaign before March.

Sanders, the lone 2020 prospect who ran in 2016, has interviewed close to 20 people to fill senior positions such as campaign manager and directors of policy, communications and his field program. A senior aide said the hiring process has been particularly focused on adding diversity — in both gender and race — that didn’t exist in his last presidential campaign.

The aide said allegations of sexual harassment between Sanders’ former campaign staff had no impact on the timeline of an announcement, which is not expected before the end of the month. Despite his challenges, Sanders is sitting on a war chest of roughly $15 million and an active nationwide network. In a show of early force, liberal activists hosted hundreds of house parties across the nation on Saturday to cheer on a second Sanders run.

Meanwhile, O’Rourke, 46, is taking steps toward a run, but an aide said he’s not expected to announce until next month at the earliest. However, he’s traveling outside Texas to introduce himself to voters in the coming weeks. Oprah is scheduled to interview him in New York City next month.

The first states on the primary calendar are not on O’Rourke’s immediate itinerary, but that’s not stopping supporters in Iowa and South Carolina from launching draft efforts. A leader of South Carolina’s “Draft Beto” movement, former Democratic National Committee member Boyd Brown, said he’s having conversations with Democratic donors, local officials and potential staffers, to help stave off commitments to other candidates as the field starts to take shape.

“We might be taking a shot in the dark, but we’re building an apparatus that we can hand off to an actual campaign should he run,” Brown said in an interview. “We’re treating this like a presidential campaign until told otherwise.”

A more seasoned political star, Biden remains silent about his plans. The 76-year-old Democrat has done little to build teams on the ground in key states, instead sticking to the schedule of huddling with aides while he moves closer to a decision.

While it may seem early, the sheer size of the likely field makes it difficult for some candidates to wait much longer. Upward of two dozen high-profile Democrats could run for a chance to deny President Donald Trump a second term.

The first Democratic primary debate will take place in June, while the first primary contest is likely a year away. With a field this big, there’s only so many donors and top staffers to go around.

“At this point the cycle, it’s a race for money and talent, and unless your name is Joe Biden or Beto O’Rourke, you’ve gotta get in soon if you want attract either one of those things,” said former Obama strategist Stephanie Cutter, who has been offering advice to some contenders.

Gillibrand, 52, joins some of her Senate colleagues on a more aggressive timeline. She’s already identified a likely location for a campaign headquarters, added staff and planned trips to key states.

The Democratic senator is eying a headquarters in Troy, New York, a small upstate city on the Hudson River. Gillibrand, who has made headlines with her work against sexual violence — and occasional profanity in public speeches — is scheduled to make her Iowa debut next weekend.

Booker is expected to run his campaign from Newark, where he lives and served as mayor. The 49-year-old Democrat has identified a likely campaign manager, Addisu Demissee. The Democratic operative previously managed Booker’s special election campaign in 2013 and subsequently worked for Hillary Clinton’s presidential campaign and most recently led California Gov. Gavin Newsome’s successful gubernatorial bid.

Like Gillibrand and Booker, Harris is expected to join the race in the coming weeks.

The first-term senator and former California attorney general has broadened her national profile in recent days by launching a tour to promote her book, “The Truths We Hold.” Like her would-be competitors, the 54-year-old Democrat has not publicly committed to running, but she has said repeatedly that the country needs a leader who “provides a vision of the future of the country in which everyone can see themselves.”

A handful of lesser-known candidates have already entered the race.

Hawaii Congresswoman Tulsi Gabbard, 37, announced her intention to run in a CNN interview that aired on Saturday. That same day, former Obama housing chief Julian Castro formally kicked off his campaign Saturday in his San Antonio hometown.

Not to be forgotten, Ohio Sen. Sherrod Brown and Minnesota Sen. Amy Klobuchar are openly considering joining the 2020 contest, although neither is as far along in preparations as their potential rivals.

Meanwhile, Washington state Gov. Jay Inslee is scheduled to make his first New Hampshire appearance next week. He talked up his liberal record and willingness to take on Trump during a weekend appearance in Nevada.

“He cannot stop us,” Inslee said of the president. “He has not stopped me, either.”

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Peoples reported from New York.

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AP writers Thomas Beaumont in Des Moines, Iowa; Will Weissert in Austin, Texas; Bill Barrow in Atlanta; and Elana Schor and Juana Summers in Washington contributed to this report.