NASA said there's a problem with a fuse box. Last week's launch had to be scrubbed because of a faulty heating unit. The new launch date is set for May 8.
Endeavour's team is led by Mark Kelly, who is married to Arizona Congresswoman Gabrielle Giffords who was shot in the head while meeting constituents. She was one of 13 people wounded in the Jan. 8 shooting in Tucson. Six others were killed.
Giffords flew to Florida last week to watch the launch in person. She has now gone back to Texas to resume her recovery at a Houston rehab hospital where she's been undergoing treatment.
A spokesman for Giffords said that she plans to be back on launch day.
The Associated Press contributed to this story