The Senate adds two new members for each state. The House has set a limit of 435 members since 1913, so no new members have been added except for 1959 until reapportionment was completed in 1963. Hawaii and Alaska each had 1 seat until then for a temporary House of 437.

At the time, the Democrats controlled Congress with solid majorities. Adding 4 new senators didn't alter the balance of power. Take a look at the reapportionment - California gained 8 House seats and Florida 4, while Michigan gained 1, reaching it's political zenith. Illinois and New York both lost seats. These changes had more of an effect than Alaska and Hawaii joining the union.