Mr. Gillum, by contrast, has offered himself as a candidate of the left. A firebrand on the stump, he has called insistently for a rollback of Republican education policies and for aggressive action against climate change. Since House Republicans passed a bill that would gut the Affordable Care Act, Mr. Gillum has made protecting health care regulations on the state level a centerpiece of his message.

“There’s muscle memory that’s been built up over a long time about what the candidate has to look like, sound like, where they have to come from,” he said. “In our case, in Florida, it hasn’t worked.”

That sense of frustration among black Democrats parallels, in some respects, the exasperation Democrats in general have voiced after the 2016 election.

If certain black candidates like Mr. Gillum and Ms. Abrams are urging an untested path, they may find primary voters more receptive to the idea after the failure of conventional Democratic strategies against Mr. Trump. Mr. Gillum and Ms. Abrams have already attracted significant interest from national liberal donors: Mr. Gillum’s first fund-raising report showed contributions from members of the Soros family, and several donors supportive of Ms. Abrams are expected to create a multimillion-dollar committee to advocate her election, according to people briefed on their plans.

The determination to compete in 2018 may run deeper in the black community, where the sense of political exclusion is even more acute. With the end of the Obama administration, there are few black Democrats in senior positions of power: just two black Democrats in the Senate and no black governors of either party. A third black senator, Tim Scott of South Carolina, is a Republican.

There is limited optimism among African-American Democrats that national party leaders will work aggressively to change that. Several of the most promising black candidates or would-be candidates — including Mr. Gillum, Ms. Abrams and Mr. Jealous — are likely to face contested primaries against well-known, well-funded white opponents. And the battle for control of the House and Senate is likely to be fought largely in rural states with few minority voters, and in suburban congressional districts where right-of-center whites often cast the decisive votes.

Symone D. Sanders, a spokeswoman for Senator Bernie Sanders in the 2016 presidential campaign, said she was concerned that national Democrats might reflexively favor white candidates in close races, rather than trusting black candidates to win over swing voters.