In Saturday's NYT, historian Sean Wilentz plays the great counterfactual of our time, "What if Kennedy Had Lived?" According to Wilentz:
"There's no question that Johnson was able to carry forward Kennedy's domestic agenda because of the 37 House seats gained by the Democrats in the 1964 elections, a landslide that produced a working majority for progressive legislation for the first time in a quarter century. But Kennedy was a more popular figure than Johnson. Had Kennedy lived to run against Barry Goldwater, the Democrats probably would have picked up 50 more liberal legislators."
Is there even the slightest bit of evidence for this? LBJ won in 1964 with 61% of the vote. This is the highest for any presidential candidate, before or since, so it seems unlikely that JFK would have improved on this by enough to carry an additional 13 seats.