CREATE TABLE table_204_400 (id DECIMAL, "district" DECIMAL, "senator" TEXT, "party" TEXT, "caucus" TEXT, "first elected" DECIMAL, "counties represented" TEXT)

which party has the most members ?
SELECT "party" FROM table_204_400 GROUP BY "party" ORDER BY COUNT("senator") DESC LIMIT 1
SELECT "party" FROM "table_204_400" GROUP BY "party" ORDER BY COUNT("senator") DESC NULLS LAST LIMIT 1