(Bloomberg) -- Joe Biden returned to the campaign trail with a new drive to contrast his views with those of other Democratic presidential hopefuls after a rough debate performance last week highlighted his tenuous grasp on front-runner status.After spending the first two months of his campaign focused on attacking President Donald Trump, the former vice president turned his attention to rivals in his own party. He began a two-day swing across Iowa to make his case for moderation as other leading candidates pull the party leftward.Biden zeroed in on health care Wednesday, arguing that Democrats should be concentrating on improving the Affordable Care Act instead of replacing it with an entirely new system such as Medicare for All. “I fundamentally disagree with anyone who says, ‘Scrap Obamacare,’” he told voters in Waterloo, at his first public appearance since last Thursday’s debate. “I’m against any Republican who wants to scrap it, I’m against any Democrat who wants to scrap it.”At a campaign stop in Iowa on Thursday, Biden said he wouldn’t respond to attacks from rivals by digging up controversies from their pasts. During the debate last week, California Senator Kamala Harris criticized him in deeply personal terms about his record of opposition, when he was in the Senate, to federally mandated busing to desegregate schools.“I’m not going to go back and use the same tactic they’re trying to use,” he said in Independence, where he walked in a Fourth of July parade. “I’m not going to go back and talk about the record of anyone from 10, 20, 30 years ago. There’s a lot out there that a lot of people would like to do differently than they did but everything is lost in context.”Asked if he understood that the conversation about race has changed, he said: “Absolutely, positively I do. As much or more than anyone.”On Wednesday, members of Biden’s communications team had dueled with Harris’s campaign on Twitter about the busing issue.Pressed on whether she’d support such a mandate for busing now, Harris told reporters in West Des Moines, “I believe that any tool that is in the toolbox should be considered by a school district.”Biden deputy campaign manager Kate Bedingfield tweeted in response that “it’s disappointing that Senator Harris chose to distort Vice President Biden’s position on busing — particularly now that she is tying herself in knots trying not to answer the very question she posed to him!”Harris’s national press secretary Ian Sams responded by saying that Biden had called busing an “asinine concept” and the back-and-forth continued with Sams later adding that he and Harris both thought Biden is “a good guy” and “That’s why a simple ‘working with segregationists to stop busing 40 years ago was wrong, and I shouldn’t have done it’ would be welcome.”He was referring to Biden’s comments from last month about two fellow senators in the 1970s who were advocates of segregation.Praising ObamacareOn Wednesday, Biden pointed out that many presidents before Barack Obama had tried to pass a health care bill and not succeeded. “We have to finish the job and make health care a right not a privilege,” he said.He has not yet rolled out his full health care plan, but suggested that he would favor a hybrid public-private system that could help the uninsured get coverage.“We can’t start over. There’s no time to start over in my view,” he said. “Building on what we’ve got, not starting over. Allowing people to keep their employer-based insurance or their private insurance or any insurance they have if they want. But if they don’t, allowing them the ability to buy into a public option, a health care plan like Medicare.”Biden’s shift to a more direct comparison with other candidates for the Democratic nomination comes as his poll numbers have fallen.A Quinnipiac national poll released Tuesday found Biden slipping to 22%, just 2 percentage points ahead of Harris at 20%. Senator Elizabeth Warren was next with 14% then Senator Bernie Sanders with 13%. A CNN national poll found Biden slipping 10 percentage points in one month — to 22%. Behind him was Harris with 17%, 9 percentage points higher than in CNN’s previous poll, and Warren with 15%, an 8-percentage-point gain, and Sanders with 14%, a 4-percentage-point drop.And a Focus on Rural America poll released on Wednesday showed a significant drop for Biden in Iowa and that he was essentially tied with Warren and Harris. David Binder Research, which conducted the survey, also works for Harris’s presidential campaign but began the poll before it was hired by Harris’s team.Biden’s campaign said Wednesday that it raised $21.5 million in the second quarter, second to Mayor Pete Buttigieg of South Bend, Indiana, who took in $24.8 million.Biden’s team was quick to note that he entered the race on April 25, nearly a month into the quarter, meaning that he took in more per day than any other candidate. Sanders said that his campaign raised $18 million during the second quarter. Harris and Warren have not yet disclosed their fundraising totals.(Adds Biden campaign stop Thursday in fifth through seventh paragraphs.)To contact the reporter on this story: Jennifer Epstein in Waterloo, Iowa at jepstein32@bloomberg.netTo contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Max Berley, Scott LanmanFor more articles like this, please visit us at bloomberg.com©2019 Bloomberg L.P.