Even with a double-digit lead before the primary, she failed to avoid the type of negative tactics that could damage her in the long haul. A new Washington Post-ABC poll says that nationally, Mrs. Clinton’s margin over Bernie Sanders has shrunk: she polls at 49 percent compared with 42 percent for Mr. Sanders; in January her lead was more than double that. If she hopes to unify Democrats as the nominee, trying to tarnish Mr. Sanders as she did in Michigan this week is not the way to go.

Mrs. Clinton’s falsely parsing Mr. Sanders’s Senate vote on a 2008 recession-related bailout bill as abandoning the auto industry rescue hurt her credibility. As soon as she uttered it in Sunday’s debate, the Democratic strategist David Axelrod registered his dismay, tweeting that the Senate vote wasn’t explicitly a vote about saving the auto industry. Even as reporters challenged her claim, she doubled down in ads across the state. As The Washington Post noted, “it seems like she’s willing to take the gamble that fact-checkers may call her out for her tactic Sunday — but that voters won’t.”

Though Mrs. Clinton spent more time in Genesee County, home to Flint, than Mr. Sanders, she only won narrowly there. Mr. Sanders’s full-throated call for top-to-bottom government accountability for Flint’s drinking-water catastrophe contrasted with Mrs. Clinton’s tepid remarks about the need for a housecleaning at the Obama administration’s Environmental Protection Agency.

Mr. Sanders’s opposition to free-trade agreements resonated in Michigan, and are likely to help him in Ohio, Missouri and Illinois next week. He has consistently pointed out Mrs. Clinton’s past support for trade pacts, starting with Nafta when her husband was in the White House, and her shifting positions ever since. Mrs. Clinton is now opposed to the Trans-Pacific Partnership, which she promoted as secretary of state. If she hopes to convince skeptical rust-belt voters that she’s in their corner, she needs to explain why she once believed that trade pacts would help American workers.

The Clinton machine should stop trying to tie Mr. Sanders to the National Rifle Association. Though Mr. Sanders has a D-minus from the N.R.A., in Michigan Mrs. Clinton’s operatives took to Twitter touting the N.R.A.’s tweets supporting Mr. Sanders’s statement that making manufacturers liable for gun violence would destroy gun manufacturing in America. On Tuesday, her campaign issued a news release saying that the mothers of Trayvon Martin and Jordan Davis, two African-American shooting victims, “are speaking out about Senator Bernie Sanders’ comments on guns and African-Americans in Sunday’s Democratic primary debate.” Mr. Sanders, like Mrs. Clinton, has spent decades working against racial discrimination, poverty and gun violence. To suggest otherwise is wrong.