Obama 353
Palin 185
Feingold 0
Feingold keeps Obama back from a landslide victory.
It depends on Obama's approvals, but you're joking if you think he still wins the election by that large of a margin if there are two liberals running. Feingold would most certainly split the left vote in Wisconsin. No way Obama gets 40% there with him on the ballot.
First of all, the OP didn't specify the year or the conditions. So I went under the assumption that it was 2012 and that Obama was moderately successful. Chances are that a Feingold run would not be able to leach off a whole lot of a fairly popular incumbent president's support.
If Obama is unpopular, then Feingold would certainly doom his chances.