WATERLOO, Iowa — 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 this week, 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 week’s debate. “I’m against any Republican who wants to scrap it, I’m against any Democrat who wants to scrap it.”

On Twitter, meanwhile, members of Biden’s communications team were dueling with Kamala Harris’s campaign about the issue that tripped up Biden during the debate — when she challenged his record of opposition, when he was in the Senate, to federally mandated busing to desegregate schools.

Pressed on whether she’d support such a mandate 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.

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 this week found Biden slipping to 22 percent, just 2 percentage points ahead of Harris at 20 percent. Sen. Elizabeth Warren was next with 14 percent, then Sen. Bernie Sanders with 13 percent. A CNN national poll found Biden slipping 10 percentage points in one month — to 22 percent. Behind him was Harris with 17 percent, 9 percentage points higher than in CNN’s previous poll, and Warren with 15 percent, an 8-percentage-point gain, and Sanders with 14 percent, a 4-percentage-point drop.