The American National Election Studies (ANES) started conducting election studies in 1948. After their 2012 study, the ANES compiled all of the studies into a timeseries. Part of their studies included "thermometer" questions about certain groups or figures where they asked respondants to rate the specific categrory on a degree scale from 0-100.
Ratings between 50 degrees - 100 degrees mean that the respondant feels favorably and warm toward the group; ratings between 0 and 50 degrees mean that the respondant doesn't feel favorably towards the group and that he or she doesn't care too much for that group. If the respondant doesn't feel particularly warm or cold toward a group he or she would rate them at 50 degrees.