How do you prevent the president from dying in Fallout New Vegas?

Answer 1

Firstly, during the security sweep, check the computer in the Hoover Dam main building. You will find evidence of an unauthorised login. Check the maintenance closet accessed from the same room, you will find a bloodstain which can be examined. You can pickpocket the engineers, and find a detonator trigger on one of them, which can be reported to the security chief. If you fail to do this, he will plant an explosive on the president's vertibird, though you have chance to disarm this or inform the security chief if you discover it.

Secondly keep an eye on the ranger at the top of the opposite tower. If you watch, a second figure will emerge and throw the first off to his death, he will be lying in front of the entrance. You can ascend and speak to the second person, who is a Legion assassin in disguise. He will evade any attempt to raise the alarm, and will try to kill you.

Lastly sometimes a man in the crowd will simple draw a switchblade and try to stab the president - this guy tends to get taken down by the rangers before he can do much damage. I believe the way to uncover him is to ask the dog handler if you can borrow the dog to help you on your sweep, it will identify the infiltrator.

Answer 2

When you exit the Hover Dam after they say President Kimball is there you exit the door with the NCR ranger and wait look at the tower behind the president and you will see a a ranger that looks odd run to the tower and kill the legion member then call it in