Levi Johnston Changes His Tune ... Hey Bristol, I'm Here To Help

Bristol Palin just got some support from her nemesis -- the baby daddy fighting her in a nasty custody war ... former fiance, Levi Johnston.

It's a strange twist. Levi, who shares 6-year-old Tripp with Palin, tells TMZ he and wife Sunny are going to do what they can to help raise her new kid. He says she's "a great mom."

And get this ... Levi says he's sorry for Bristol's string of bad luck, specifically mentioning the fact she called off her wedding.

Palin posted a somber announcement on her Facebook page Thursday and noted that the pregnancy was a "huge disappointment" to her and her family, but Sunday she said the pregnancy was planned.

As for the whole kid out of wedlock thing, Levi says waiting to have kids until the parents tie the knot is the way to go, but he understands things happen ... they happened to him.