This is markedly different from the approach that party leaders have taken over the last eight years, when President Obama defined the party from top to bottom with his personality and policies. Instead, Democrats intend to focus on a sparer agenda of bread-and-butter priorities that can win support from both liberal and moderate officeholders — and appeal to voters just as much in red states as along the two coasts.

Beyond that, they expect wide variance in how officeholders handle Mr. Trump and his agenda, from moderates who seek out accommodation to blue-state leaders who pursue total war. Their emerging message is likely to focus on protecting Medicare and Social Security, attacking income inequality and political corruption, and blocking legislation that might restrict access to health care.

The first salvo in the fight will be over Mr. Trump’s pick of Representative Tom Price of Georgia, a vocal supporter of privatizing Medicare, as secretary of health and human services: The Democrats at Ms. Heitkamp’s dinner discussed how to highlight and, potentially, block Mr. Price’s appointment, according to an attendee.

Many Democrats, especially those from the party’s more liberal wing, see significant reasons for optimism about their political prospects in the medium and long terms. Though Mr. Trump prevailed in the Electoral College, he did so thanks to the slimmest of margins in three states — Michigan, Pennsylvania and Wisconsin — and he lost the popular vote nationally by nearly three million votes.

With the country growing more urban and racially diverse, Democrats like their chances in the 2020 presidential race, when nonwhite voters are likely to make up more than 30 percent of the electorate for the first time in history. Even in Mrs. Clinton’s defeat, the country’s changing hue helped her come closer than any Democrat in decades to flipping Arizona and Georgia, two traditional Republican bastions.

But before the next presidential race, Democrats must navigate a treacherous landscape of midterm elections. They must defend 10 Senate seats in states Mr. Trump won, many of them in the Rust Belt and the West. And unless they break the Republican lock on state governments by winning governorships in a handful of big states, like Wisconsin and Ohio, they may be unable to prevent Republicans from continuing to draw congressional maps that favor their candidates.