KFF's findings may give pause to Democratic presidential candidates preparing for the second round of primary debates, scheduled for Tuesday and Wednesday evenings on CNN.

Asked to choose between building on the ACA and replacing it with a national Medicare for All plan, 55 percent of Democrats and Democratic-leaning independents said they would expand the existing law. By comparison, 39 percent said they would prefer replacing the law with Medicare for All.

Respondents also voiced strong support for employer-based insurance, a reality likely to complicate efforts to replace the existing system. More than three-quarters have a favorable opinion of employer-based insurance, with an overwhelming majority of those covered by such plans rating their coverage as “good” or “excellent.”

Nearly all respondents covered by Medicare rated their coverage as “good” or “excellent.”

Primary season is when most candidates trot out the proposals that appeal to their party's more ideologically driven voters, who are kicking the tires on a slate of potential nominees. Support for Medicare for All has become a litmus test for many progressive voters as they contemplate their more than two dozen candidates.

But as the primary season wanes, the remaining candidates traditionally shift toward more incremental proposals, hoping to attract the moderate voters they may need to win the presidency.

The new KFF poll shows the public's support for the idea of Medicare for All has dipped to about 51 percent, from 56 percent in April.

In recent years, Medicare for All saw the peak of its favorability in March 2018, when about 59 percent of the public said they favored a system in which all Americans would get their insurance from one government plan.

While the newest numbers show support shifting among both Republicans and Democrats, the share of Democrats who said they “strongly favor” a Medicare for All plan has dropped to 42 percent, from 54 percent in April.

Medicare for All's slip in popularity has come as Democratic presidential candidates have shared the details of their plans and Republicans have tested out campaign messages about creeping “socialism” in the health care system, suggesting an uphill battle toward making such a plan a reality.

About 83 percent of Democrats and Democratic-leaning independents said it is “very important” for the candidates to discuss health care during this week's debates, with half saying they would prefer the candidates focus on the differences among themselves rather than with Trump.

The poll also found strong support for some of the ACA's key provisions, including among Republicans, as a challenge led by GOP state officials and endorsed by the Trump administration winds through the federal court system.

For example, more than 7 in 10 respondents said it is “very important” to prevent insurers from denying coverage to those with preexisting conditions, while 64 percent said it is “very important” to prevent them from charging sick people higher premiums than they charge healthy people.

Conducted July 18-23, the KFF poll surveyed 1,196 adults by landline and cellphone, in English and Spanish. The margin of error is plus or minus 3 percentage points.

Kaiser Health News is a nonprofit news service covering health issues. It is an editorially independent program of the Kaiser Family Foundation, which is not affiliated with Kaiser Permanente.

Read more: 

• How much would a public option cost?
• 4 approaches to expanding health care (and only one is Medicare for All)
• One employer makes a case for Medicare for All