Not everyone agrees: Some argue that just focusing on Mr. Trump will not be enough to mobilize an increasingly diverse and liberal party.
To win, they argue, Democrats must energize the minority and young voters who have moved to the left on social and economic issues. Those voters, they say, want to hear more than simply a critique of a president they already believe is unfit for the White House.
“That is not the sole message and it cannot be because there are real challenges in the country that predate Trump,” said Adrianne Shropshire, the executive director of BlackPAC, an organization that works closely with African-American voters.
Hoping to inspire primary voters, many of the leading candidates have embraced proposals that were once constrained to the fringes of the party. They have pitched eliminating most of the private health insurance industry, expanding the size of the Supreme Court, decriminalizing illegal border crossings and embracing reparations for slavery to some black Americans.
Their campaigns point to polls showing that majorities of Americans agree. A New York Times/SurveyMonkey poll found that two-thirds of Americans and 81 percent of Democrats back a 2 percent tax on households with a net worth of more than $50 million, as Ms. Warren proposed. The same survey found that 59 percent of Americans support free tuition at public colleges, and 58 percent support Medicare for All — both of which have long been staple policies for Mr. Sanders.
But when asked about the specifics of those plans — like eliminating private health insurance — those numbers often drop.
Moderates worry that images from the primary debates, like the photos from the June debate that show 10 Democratic candidates raising their hands in support of free health care for undocumented immigrants, will come back to haunt the candidates in the general election.
“There are some things that some of these candidates want that are not achievable and will probably cost Democrats the election,” said Danny Homan, the president of AFSCME Iowa Council 61, the state’s largest employee union. “Let’s not go so far left that normal, average, everyday citizens say, ‘That’s not for me.’”