Billionaire Tom Steyer pledges $12 million to support Proposition 50, which could give Democrats five more congressional seats in California. Charles Munger Jr. has donated $32 million to the opposition, arguing that the measure undermines California’s voter-approved independent redistricting commission. As California voters receive mail ballots for the November special election, which could upend the state’s congressional boundaries and determine control of the House, billionaire hedge-fund founder Tom Steyer said Thursday he will spend $12 million to back Democrats’ efforts to redraw districts to boost their party’s ranks in the legislative body. The ballot measure was proposed by Gov. Gavin...