(CNN) The decision by Bernie Sanders' campaign to immediately engage with Joe Biden -- in a series of public remarks, tweets and emails to supporters -- came from the top, campaign manager Faiz Shakir told CNN on Thursday.

"This was driven by Sen. Sanders himself," Shakir said in an interview. "He said, 'Why the heck should I wait to draw contrast between the two of us? That is what a primary is all about.'"

Since Biden entered the presidential primary last week, Sanders has repeatedly sought to highlight where he and the former vice president have split on big-ticket policy questions -- especially those centered on trade and other economic issues. Both candidates, despite their opposing records and differing views, are appealing now to the same base of voters: working class Democrats and independents in the Midwestern states President Donald Trump nearly swept in 2016.

Shakir conceded that Biden is the frontrunner -- polls have shown him building on his early lead since joining the contest last week -- and said Sanders and the campaign expected Biden to come out of the gates strong. But they were also determined to define Biden early on by shining a spotlight on the pieces of Biden's record that, though perhaps standard for Democrats at the time, might run against the party's more recent progressive shift.

A spokesperson for Biden's campaign declined to comment.

"On many issues, Vice President Biden has been wrong first," Shakir said. "In some cases (like the war in Yemen) he has course corrected. In others (like trade) he maintains the wrong position."

According to Shakir, Biden's position on trade will continue to be a point of special emphasis for Sanders, who has been bothered by his rival's efforts to present himself as a "union man," while still -- as he did this week -- defending his support for NAFTA, the North American Free Trade Agreement.

Biden in Iowa said he did not regret his vote for NAFTA. Sanders opposed it and has called on President Donald Trump to "go back to the drawing board" on the United States-Mexico-Canada Agreement (USMCA), his administration's renegotiated version of the deal.

"I think when people take a look at my record versus Vice President Biden's record -- I helped lead the fight against NAFTA; he voted for NAFTA," Sanders said Monday night in an interview with CNN's Anderson Cooper. "I helped lead the fight against (permanent normal trade relations) with China; he voted for it. I strongly opposed the Trans-Pacific Partnership; he supported it. I voted against the war in Iraq; he voted for it."

On Wednesday, Sanders responded on Twitter to Biden's comment earlier in the day when -- in pointing out China's own assorted internal problems and divisions -- said the country is "not competition for us."

"Since the China trade deal I voted against, America has lost over 3 million manufacturing jobs," Sanders tweeted. "It's wrong to pretend that China isn't one of our major economic competitors. When we are in the White House we will win that competition by fixing our trade policies."

Biden's campaign on Wednesday evening sought to clarify his remarks, which drew criticisms from Republicans like Sen. Mitt Romney.

"As he has many times, Vice President Biden underscored that, whatever challenges we face as a nation, including those posed by a rising China, they pale in comparison to the structural and social challenges that confront China itself," Andrew Bates, a Biden spokesman, said in a statement.

While Sanders was spearheading the strategy, Shakir said, he made it clear to his team that he wanted the distinctions to be made only on policy issues and not veer into what could described as personal attacks, emphasizing often during internal campaign conversations that, "I like Joe, we are friends."

But those warm feelings will only go so far as the race heats up. During the first 24 hours of Biden's campaign, Sanders sent out a pair of fundraising emails to supporters, both naming the former vice president in the subject line and signed by Shakir.

"It's a big day in the Democratic primary and we're hoping to end it strong," the second began. "Not with a fundraiser in the home of a corporate lobbyist, but with an overwhelming number of individual donations in response to today's news."

Sanders' argument, Shakir said, is meant to address both the issues and questions of electability. While Biden may be prepared to shift his positions on several key issues, Shakir argued that voters will want to know where a candidate was to begin with -- and would be turned off by a candidate they can't nail down.

"A track record matters," Shakir added, "especially in these places that are the pathway to the presidency. Voters in places like Michigan and Pennsylvania want to know that someone has been fighting for them from the beginning. This is not about re-litigating the past. It is about what the past can tell you about the future."

Shakir also said the Sanders team is prepared to place a special emphasis on the differences between the two candidates when it comes to foreign policy. He pointed to Sanders' signature achievement on that front: leading the push on Capitol Hill to use the War Powers Act as a means of compelling the White House to end the US involvement in the Saudi-led war in Yemen.

A spokesperson for Biden announced the former vice president's support yesterday for a congressional override of Trump's veto of that legislation -- a decision that effectively condemned the policy initiated during his time serving under President Barack Obama during the last administration.

On Thursday afternoon, Sanders in a tweet from his Senate account, thanked Biden for "joining" him.

"I thank my friend @JoeBiden for joining our effort to end U.S. involvement in the Saudi military campaign in Yemen, which, over four years of war, has experienced the world's worst humanitarian crisis," Sanders said. "We must override @realDonaldTrump's veto."

"Switching his position on big issues like these will force voters to ask 'what are (Biden's) core convictions?'" Shakir said. "If that is where he was then and this is where he is now -- where will he be in the future? We can always say that is where Bernie has always been."

CORRECTION: This story has been updated to identify the day Sanders tweeted about China and the timing of the Biden campaign's statement clarifying his remarks on China.