Backers of California state Sen. Kevin de León, a Democrat challenging Sen. Dianne Feinstein (D-Calif.) from the left, are tiptoeing into the June 5 primary with their first TV ad. The 60-second spot from A Progressive California, a five-figure buy designed to run over one day in Los Angeles media, introduces de León by way of his speech at the California Democratic Convention — where Feinstein’s failure to win the official party endorsement made national news.

The small scale of the buy points to de León’s challenge. The 51-year-old Democrat has represented part of Los Angeles in California’s legislature since 2006. He has struggled to raise his name recognition across the state, however, and has polled no higher than 24 percent against Feinstein since entering the race. Feinstein ended 2017 with around $9.8 million in the bank; de León ended it with less than $400,000.

That’s where the super PAC comes in. Backed by the California Nurses Association, one of the state’s largest unions, the PAC’s first buy is designed to show that there will be enough money to promote de León into the runoff. (The CNA’s affiliate, National Nurses United, was the biggest independent backer of Sen. Bernie Sanders in his 2016 presidential bid.)

California’s primary will set up a November runoff between the two top finishers. While Republicans have failed to recruit anyone considered a strong challenger to Feinstein, liberals do not want to risk de León falling out of the top two spots.