He was ineffective because he would never turn the screws on his opponents. When you deal with people who think the game is winner take all, then you need to take everything from them when they refuse to play. He would never even threaten that level of damage to his opposition.
The Navy service notwithstanding, he was no warrior. No killer instinct. Heck, he didn't even dispatch the "killer rabbit" that was trying to get in his boat. :)
but incumbency has its inertia and Chappaquidick Ted did not have much of a chance on the national stage. The differences on national health care approaches between the two survive to this day. Carter still hasn't let go of that because it was a missed opportunity causing a 30 year delay of Cartercare. :)