Tesla will soon need money again. The trouble is, raising it suddenly looks a lot more challenging.

Moody’s Investors Service downgraded Tesla’s debt on Tuesday afternoon, citing persistently negative cash flow and continued production issues with the Model 3 mass-market sedan. Moody’s is keeping a negative outlook on the credit due to “the likelihood that Tesla will have to undertake a large, near-term capital raise in order to refund maturing obligations and avoid a liquidity shortfall.”

Tesla’s...