How a New York Democrat Lost a Progressive Ballot Line to a G.O.P. Proxy

When a former Democratic congressman running for his old House seat broke ranks last month and endorsed the primary opponent of Representative Jamaal Bowman in a neighboring district, his longtime allies in the left-leaning Working Families Party were furious.

They declared that the former congressman, Mondaire Jones, had “strayed from the values and principles that made us proud to support him,” and canceled plans to actively campaign for his New York comeback bid this fall.

Then things took an even more bizarre turn.

The Working Families Party held what should have been a drama-free primary, but instead of nominating the liberal Mr. Jones, a final tally this week showed that voters overwhelmingly chose the unlikeliest of alternatives as their nominee: a bankrupt local businessman propped up by Republicans.

The story behind the embarrassing upset could hardly be better scripted for a turbulent era in New York politics, complete with Democratic infighting, a starring role by the state’s fast-growing Orthodox Jewish community and Republican meddling that are still coming into view.

But the potential consequences are farther reaching. Mr. Jones’s race against Representative Mike Lawler, a Republican in the Hudson Valley, is a must-win for Democrats trying to retake the House, and the loss of a coveted third-party line could prove the difference in what is expected to be a tight race.

“There are people who won’t vote on the Democratic line for ideological reasons, but they will vote for the same candidate on the W.F.P. line,” said Jennifer Cabrera, a leader of the party’s local chapter. “It’s a very real thing outside New York City.”