Potential Democratic presidential candidates are campaigning for the midterms in states that President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE took from their party’s column in 2016, testing the waters for the next presidential election.

The so-called invisible campaign has begun with would-be candidates including Sens. Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (D-Calif.), 53; Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.), 49; and Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.), 68, checking out the political landscape while campaigning for congressional candidates.

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“It’s like preseason football practice: you’re not playing in pads yet but you’re on the field, leading the playbook,” said David Wade, a Democratic strategist who served as a senior aide to 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 during his presidential bid in 2004, said of the presidential hopefuls. “It’s candidate training, with a net.”

A number of presidential hopefuls have been stumping in blue states that turned red during the 2016 presidential election, such as Wisconsin, Michigan and Pennsylvania. Trump was the first GOP presidential candidate to win Wisconsin since 1984, and the first to win the other two states since 1988.

Harris, for example, attended a fundraiser for Sen. Debbie Stabenow Deborah (Debbie) Ann StabenowOVERNIGHT ENERGY: Trump rollbacks could add 1.8 billion tons of greenhouse gas emissions over 15 years: analysis | Intensifying natural disasters do little to move needle on climate efforts | Experts warn wildfire smoke could worsen COVID-19 GAO report finds brokers offered false info on coverage for pre-existing conditions Democrats back away from quick reversal of Trump tax cuts MORE (D-Mich.) last month and sources close to her say she is expected to campaign for incumbent Sen. Tammy Baldwin Tammy Suzanne BaldwinKeep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser Democrats seek balance in backing protests, condemning violence MORE in Wisconsin in the coming weeks.

Baldwin also recently received a boost from Booker, who appeared at two events in Madison and Milwaukee. He has also been making other appearances, including in Florida, where he campaigned for Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE, and in Ohio for Sen. Sherrod Brown Sherrod Campbell BrownSenate Democrats release report alleging Trump admin undermined fair housing policies Bipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda MORE.

Warren has had a similar schedule, campaigning in Ohio, Wisconsin and Michigan for her Senate colleagues. In addition, she did a fundraiser for Brown and also campaigned with Ohio gubernatorial candidate Richard Cordray Richard Adams CordrayConsumer bureau revokes payday lending restrictions Supreme Court ruling could unleash new legal challenges to consumer bureau Supreme Court rules consumer bureau director can be fired at will MORE, who led the Consumer Financial Protection Bureau she spearheaded, last month in the Buckeye State. Meanwhile, Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) has stumped for Democratic candidates in states including Iowa, Pennsylvania, Wisconsin and Arizona.

Former Vice President Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE, 75, who has said he is considering a 2020 run, has campaigned in red states including Montana and North Dakota along with Pennsylvania, where he campaigned for Conor Lamb, who won a House seat in March.

“If they are smart, they are also listening, trying catch the vibe out there that Clinton, the [Democratic National Committee], and most of the rest of us so clearly missed last time around,” said Cal Jillson, a professor of political science at Southern Methodist University, said of the visits to the key states.

The midterm campaign is seen as a time for those with 2020 hopes to earn political chits while getting exposed to voters around the country. It also allows candidates to try out a version of their stump speech with very low risk to their personal brands.

“It’s an indispensable part of getting ready to run,” Wade added. “You get into a rhythm rhetorically, you spot strengths and weaknesses and candidate mechanics to work on before the invisible primary season goes public and you pick a whole universe of early state supporters who are forever loyal to the people who helped them when the chips were down.”

The visits could also highlight who is popular with voters at an early stage, as candidates fighting for attention in 2018 seek to bring in the Democratic stars that will most help them in the midterms.

To date, it seems like anyone’s race.

“Every cycle, this is the first big indicator about who is wanted out on the campaign trail and who is likely to get attention in the months ahead,” one top Democratic strategist said. “You begin to start seeing a field come into view.”

The campaigning can bear fruit for 2020 if the top-tier Democrats help members of their party take congressional seats and other political posts.

“A presidential hopeful who campaigns for a successful 2018 candidate can expect the winners to pay back big time in 2020,” said Democratic strategist Brad Bannon, who added that some of the winners could become superdelegates in two years.

But Bannon said there are some downfalls to campaigning for the 2018 candidates.

“The big problem is backing the wrong 2018 candidate, which could bit you on the butt in 2020,” he said.

The would-be candidates also need to be careful to not “say something stupid that could kill a 2020 campaign in the crib before it’s ready to stand by itself,” said Bannon.

Wade added that the prospective candidates need to walk a fine line when they’re out on the stump.

“A little subtlety goes a long way,” he said. “The presidential aspirants who breeze in and make themselves the show rather than the candidates on the ballot this year often end up alienating as many people as they attract. Your job is to help the party, help the candidates and remember that it’s not about you, it’s about them.”