Complex sentence: Rainey was seated December 12,1870 and was re-elected to Congress four times; he served until March 3,1879, which made him the longest-serving black Congressman prior to William L. Dawson in the 1950s.
Split into simpler sentences: Rainey was seated December 12,1870 and was re-elected to Congress four times. Serving until March 3,1879, he established a record of length of service for a black Congressman not surpassed until that of William L. Dawson in the 1950s.