Very possibly. From the Washington Post:
…in a telephone press conference with South Dakota reporters on Wednesday, Johnson said his earlier statement should have been more clear about the prospects he will seek a fourth term.
“If I run again, I will run a strong campaign is what I meant. But only if I run again, and it’s far too soon to make that statement,” Johnson said.
Tim Johnson has had multiple health issues, which – to be blunt about it – gave him a certain sympathy vote in 2008. Couple that with the monstrous anti-Republican wave of 2008 and it’s not very surprising that South Dakota voters gave him another term. But in this term he will enjoy neither advantage; Johnson usually has squeaker races; and he’ll be facing former Governor Mike Rounds, who served two terms and was quite popular. The Democrats are nervous about this one, and rightly so: even if Johnson sticks it out he’s still going to be highly vulnerable. And this latest backtrack of Johnson probably won’t reassure them much.
So I guess that we’ll see.