Atlantis will deliver a full load of spare parts to the International Space Station, along with six astronauts who will unload everything.
Liftoff is scheduled for 2:28 p.m. Monday. Although an unmanned rocket did not take off Saturday morning with a communication satellite as planned, the countdown for Atlantis is still proceeding.
That's because the Atlas rocket has a technical problem that cannot be fixed quickly. A Sunday launch attempt would have delayed the shuttle flight by one day.
Forecasters say there's a 90 percent chance of good flying weather Monday.
Only six shuttle missions remain, including this one.