Democrats are expecting one of their party’s biggest fields in history will battle to take on President Trump in the 2020 election.

They say Trump’s low approval ratings, his lack of legislative accomplishments and the lingering controversy surrounding multiple investigations into Russia’s involvement in the 2016 race have a number of Democrats positioning themselves for a White House run.

“So long as Trump is hanging around [with approval ratings] in the 40s, potential challengers will be attracted like moths to a flame,” said David Wade, a Democratic strategist who served as a top aide to former Sen. John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE (D-Mass.) in his 2004 presidential run.

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Another factor? The lack of a whale candidate who might scare off other rivals.

For the first time since Kerry was the party’s nominee, no one named Clinton or Obama is expected to run for the Democratic nomination.

“Candidates knew that opportunities didn’t come around often to run in a wide-open field without an anointed front-runner or incumbent Democratic vice president,” Wade said, comparing the lead-up to the 2004 cycle with the 2020 cycle.

Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE was the clear front-runner going into both the 2008 and 2016 campaigns, and her presence left some Democrats wary of getting in the race — particularly in the last cycle. In 2012, no Democrat challenged then-President Obama.

“Hillary and her team did a good job freezing the field and keeping most other potential candidates at bay,” Democratic strategist Jim Manley, a Clinton surrogate in 2016, said of last year’s cycle.

“The Democratic Party thought they could try and control the process, but I don’t think that’s going to be an option this time around. No one is going to be able to clear or winnow the field. There’s nothing anyone can do about it.”

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Dozens of Democrats, along with Independent Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (Vt.), are thought to be eyeing the race, though no one has officially thrown their hat in the ring at this early stage.

Besides Sanders, 75, the heavyweights include former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE, 74, and Sen. Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.), who is 68.

A survey released this week by the Democratic-leaning firm Public Policy Polling showed Biden ahead of Trump 54 percent to 39 percent in a hypothetical matchup, and Sanders leading 52 percent to 39 percent.

It also found Warren and Sens. Cory Booker (D-N.J.) and Kamala Harris (D-Calif.), who are also talked about as potential candidates, ahead of Trump — though by smaller margins.

While much can change in the coming years, Trump has just a 39.8 percent approval rating in the RealClearPolitics average of polls after less than seven months in office. Those numbers will make many Democrats think they have a chance of defeating him if they can win their own party’s nomination.

Other Democrats thought to be weighing their options include Sens. Amy Klobuchar Amy KlobucharBattle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates Klobuchar: GOP can't use 'raw political power right in middle of an election' MORE (Minn.), Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (N.Y.), and Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (Conn.), as well as Govs. Terry McAuliffe (Va.), John Hickenlooper (Colo.), Jay Inslee (Wash.) and Steve Bullock (Mont.).

“We’re seeing huge excitement out there for Democrats as we fight for policies that support American families and expand economic opportunity, while Republicans are working to strip away healthcare from millions and are led by one of the most unpopular presidents in history,” Sabrina Singh, deputy communications director for the Democratic National Committee, said in response to questions about the possible field of candidates.

People close to Trump aren’t worried about the potential competition.

They note that Trump faced a huge crowd of GOP rivals in 2016 that included former Florida Gov. Jeb Bush, Sens. Ted Cruz Rafael (Ted) Edward CruzMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (Texas) and Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (Ky.), and Ohio Gov. John Kasich.

He then beat Clinton in a stunning outcome on Election Day.

“The president defeated the largest field of Republican candidates ever, took on the Washington, D.C., establishment, at least two political dynasties and the mainstream media,” one White House official said. “Perhaps there will be another sizable group of opponents in 2020, but what is their message to the American people? What do they stand for?”

“The president’s message is clear, and he is hard at work to ensure the forgotten man and woman will never be forgotten again with ‘America first’ policies that benefit our country and its great people,” the official said.

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It’s not hard to find signs of potential candidates positioning themselves for 2020.

Earlier this month, Sanders returned to Iowa for the first time since losing to Clinton in the Democratic caucus there last year to give a keynote address sponsored by the Iowa Citizens for Community Improvement. He’ll return to the Hawkeye State again next month to promote his book, “Bernie Sanders: Guide to Political Revolution.”

Harris has attempted to expand her presence on the national stage, hobnobbing with the party’s megadonors in the Hamptons while making a name for herself in a string of intelligence hearings.

Warren, a wonk on economic policy, is now a member of the Senate Foreign Relations Committee and recently traveled to Afghanistan — experiences that would boost her foreign policy credentials.

Biden will embark this fall on a 19-city book tour, which one former adviser said will be “a barometer” for a 2020 run.

Some of Biden’s supporters think a large pool could help the former vice president if he chooses to run. “Absent an Obama-level talent to break through the rank of newbies, [he] would be the only one with a pretty comfortable 20-30 percent that knows him and feels really positive about him, which is a great asset in a big, big field,” a former adviser to Biden said.

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At the same time, Democrats have their own problems.

The Sanders primary challenge to Clinton last year exposed divides that the party is still trying to heal.

Republican strategist Doug Heye said that while there is “no shortage of enthusiasm among Democrats to take on Trump,” the party remains “in disarray and distrustful of their own party’s structure.”

“The sheer number of people looking to run could create a situation similar to what Republicans faced last cycle with so many candidates, and further expose Democrats’ multi-faceted divisions,” he said.

Of course, Republicans ended up winning the White House in that cycle.

And Democrats say they welcome the potentially large number of candidates who might vie for the presidency.

“At least for now, the more the merrier,” Manley said. “We need to have a serious competition for ideas.”