Nancy Pelosi has led Democrats to four straight defeats in elections to control the U.S. House, which explains why Republicans are endorsing her to stay as Minority Leader. She finally has a challenger in Ohio Rep. Tim Ryan in a secret-ballot vote on Wednesday, but the party’s weakness in Congress runs deeper than its leadership.

Mr. Ryan, a seven-term Congressman who represents northeast Ohio around Youngstown, has promised to bridge the party’s cultural chasm with white, working-class voters. The 43-year-old has also pledged...