Backers of a new California stem cell agency bond measure await the Nov. 6 election to see which state officeholders they’ll have to deal with to try to get tax breaks for private donors.

The California Institute for Regenerative Medicine will run out of money in the last quarter of 2019. The agency was created under 2004’s voter-approved Proposition 71 with $3 billion in bond funding.

Proponents of keeping the agency alive are seeking gap funding in advance of...