  Radio observations have resulted in some of the most fundamental and exciting
discoveries in time-domain astronomy. Here I demonstrate the potential for the
Next Generation Very Large Array (ngVLA) to localize prompt
centimeter-wavelength counterparts to the coalescing binary supermassive black
holes (SMBHs) to be detected by the LISA mission. The resulting redshift
measurements of the hosts of LISA-detected coalescences will populate a Hubble
diagram up to the epoch of the earliest galaxies. Analysis of the host
environments and the electromagnetic coalescence signatures will provide
crucial insight into the interactions between binary SMBHs and their
environments, and the mechanisms of SMBH formation and growth.
