1970 is the best chance for Republicans to win Congress. Democrats should have gained more seats in 1972 as Republicans were defending their 1966 wave seats-it's like the 2014 Senate elections vs 2016, Republicans were unlikely to lose seats in 2014 or gain any(net gain) in 2016. If a Nixon win as shown above in 1970 is implausible perhaps he can win bigger in 1968-if Republicans won all the Senate seats they lost by 4% or less they would gain 3 more seats in 1968, including defeating Birch Bayh. In 1970 the Democrats lost 4 seats; 53-4=49.