WASHINGTON (AP) _ For months, Sen. Byron Dorgan carried on a private, painful struggle.

Doctors couldn't explain why his oldest daughter died in 1994 after an operation to correct a heart defect, and he faced the possibility that his youngest daughter would need surgery for a different heart problem.

Now he says one way he's decided to deal with the pain and grief is by becoming an advocate for heart research.

``Losing a child is a devastating, devastating thing, second to nothing you'll experience in life,'' the North Dakota Democrat said at a news conference with American Heart Association officials.

``I've taken some while to try to think through and understand and evaluate how does one deal with and respond to this,'' he said. ``One way to deal with it is try to do the things that can be helpful, so other families don't go through the same kind of tragedy.''

Dorgan contacted the American Heart Association to offer his assistance and is helping form a group of lawmakers to push for increased funding for heart research. The heart association says that heart research is severely underfunded in relation to other diseases.

Shelly Dorgan, a daughter by a previous marriage, was 23 when she underwent surgery in late 1994 to correct an irregular heartbeat.

``She entered heart surgery with enormous optimism and hope, and she did not survive it,'' Dorgan said, adding that no doctor has been able to explain why she died. The death left the senator with an ``emotional void for some while,'' he said.

``I've learned that the more we know about the heart the more we know we don't yet understand.''

His youngest daughter, Haley, 6, has a hole between the chambers of her heart. The latest opinion from her doctors is that she won't need surgery, Dorgan said in an interview.

``I don't know how we'd go through another heart surgery,'' he said. ``We've just been on pins and needles.''