With a series of candidate recruiting successes, Democrats have established a plausible, if still difficult, path to recapturing the Senate this year. But to regain the majority, the party will need to overcome the blue-collar barricade.

The fight for the Senate is expanding to a few more states next year, as both parties tout talented candidates and point to political dynamics tilting in their favor. Democrats still face an uphill battle to net the four seats they need to take control of ...