Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE is plotting a series of campaign stops in purple states and districts crucial to the Democratic Party's hopes of winning back majorities in the House and Senate.

The former vice president is expected to be a constant presence in the Midwest, and is also expected to visit swing districts in states such as Florida and Nevada, according to sources.

Biden is seen as a strong surrogate for Democrats, particularly in places where they are seeking to win back territory lost to President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE in 2016.

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And that means Biden’s trips are being seen through the prism of 2020, when he could make a third run for the White House.

“You can kind of see what his strategy is not just in terms of 2018 but in terms of 2020,” said one Democratic strategist who has worked on presidential campaigns. “He’s essentially going to the places Democrats need desperately if they want to win in the midterms and the presidential election. This is very much a pre-presidential campaign campaign.”

After Labor Day, Biden is expected to campaign alongside Elissa Slotkin — a former Obama administration official running for a House seat in a toss-up Michigan race against GOP Rep. Mike Bishop.

Biden is also set to stump in central Florida for Rep. Stephanie Murphy Stephanie MurphyDemocrats scramble on COVID-19 relief amid division, Trump surprise Bank lobbying group launches ad backing Collins reelection bid House Democrats call on State Department for information on Uighur prisoner Ekpar Asat MORE, whose race is rated “likely Democrat” by the Cook Political Report. In Nevada, he’ll campaign for Rep. Jacky Rosen Jacklyn (Jacky) Sheryl RosenSenators introduce bipartisan bill to help women, minorities get STEM jobs Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections Google, Apple, eBay to meet virtually with lawmakers for tech group's annual fly-in MORE (D), who is in a neck-and-neck Senate race with Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (R-Nev.).

He’s also set to campaign with Mikie Sherill in New Jersey, who is seeking to win a House seat held by retiring Rep. Rodney Frelinghuysen Rodney Procter FrelinghuysenBottom line Republican lobbying firms riding high despite uncertainty of 2020 race Ex-Rep. Frelinghuysen joins law and lobby firm MORE (R). Cook rates the race as leaning Democrat.

Last month, he appeared at a fundraiser for Ben Jealous, the progressive candidate who is running for governor of Maryland. Before a crowd of 250 people, Biden urged Democrats to come together and defeat Republicans in the midterms.

“We have to stop them,” he said, according to the Baltimore Sun. “We can’t do it if we’re divided.”

He’s also campaigned for Rep. Conor Lamb in Pennsylvania, as well as Sens. Jon Tester Jonathan (Jon) TesterDemocrats shoot down talk of expanding Supreme Court Pence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans MORE in Montana, Heidi Heitkamp Mary (Heidi) Kathryn HeitkampHarris faces pivotal moment with Supreme Court battle Centrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents MORE in North Dakota and Amy Klobuchar Amy Klobuchar3 reasons why Biden is misreading the politics of court packing Social media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight MORE in Minnesota.

“Since 2008, Joe Biden has been a man of all markets,” said Democratic strategist Eric Jotkoff. “He is the guy you want showing up to help your campaign in Alabama or Massachusetts and he is someone who is comfortable talking to a crowd in Daytona Beach as he is in Chicago or Houston ... That is why he is in such demand as a surrogate in 2018.”

If the candidates Biden is lending a hand to are successful this fall, it could help his supporters argue that he is the best candidate to beat President Trump in 2020.

The former vice president has led a number of polls surveying Democratic voters, and he’s also performed well in polls against Trump. Most political observers see him as a credible general election candidate who can win over centrists and independents.

The bigger question is whether Biden, 75, can win over Democratic primary voters further to the left. He’s already been an also-ran in two party primaries, and would face stiff competition from the likes of Sens. Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.), Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.) and Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.).

The case Biden might make to primary voters is that he is the kind of candidate who can take back states such as Pennsylvania, Michigan and Wisconsin, which Trump took from the Democratic column in 2016.

Biden visited many of those states while promoting his book, "Promise Me Dad".

“His book was the first trial run,” the strategist said. “And the midterms are the second and final trial run before he decides whether he should run again.”

Sources close to Biden refuse to discuss any potential 2020 plans, saying the former vice president’s immediate priority is 2018. But they say he will make a decision about whether he will make a White House bid by January.

Jim Manley, the Democratic strategist, said Biden’s strategy telegraphs what he may be thinking beyond 2018.

“He’s playing the long game without being too terribly blatant about it,” Manley said. “He’s clearly focused on the states many believe we can win with the proper candidate.”

Still, Manley said he’s not convinced Biden is ultimately what the party needs.

“As we struggle to find new faces I'm not so sure someone like the former VP is what the party is looking for,” he said.

A Politico/Morning Consult poll out on Wednesday showed Trump trailing Biden by 12 percentage points. The president also trailed Sanders by the same amount, according to the survey.

The former vice president has also led in other recent polls, including a Zogby poll in June where he received the support of 21 percent of those surveyed. Sanders came in second with 19 percent.