The House Always Wins VI

 is a main quest in Fallout: New Vegas.

Quick walkthrough
Protect President Kimball

Detailed walkthrough
You must protect President Kimball from an assassination attempt. Start by proceeding to Hoover Dam and talk to the Ranger in charge, Ranger Grant. The game then forces you to wait until the next day. After talking to the ranger again he will explain the plan of the day. The quickest way to do this quest is to follow the Ranger to the observation deck, then tell the Ranger that you are ready and a Vertibird will land above you. Next, go up the ladder on the side and wait until you see an engineer. Putting the jumpsuit on will allow access without resistance.
 * If you haven't been granted full access to all areas you will need to pass a 50 point speech test to get past the guard to the ladder without turning everyone hostile. Alternatively, there is an NCR jumpsuit in a locker in the closet with the blood in, negative karma will be given if the suit is taken but is a good option if speech is low.

After he passes, examine the Vertibird and you will see that there is a bomb - if you have 50 Repair or Explosives you can disarm it, otherwise you'll need to steal the detonator from the engineer. Talk to the Ranger once more and tell him about the bomb. The president is then rushed to the Vertibird and quest is completed. If you wish, you can say nothing to hear out the entire speech, though if you do you'll need to kill the sniper that tries to take out Kimball from the tower behind his podium.

Alternatively,

You can hack into the computer behind the reception counter on the first level of the terminal building(the building on which the helipad is), behind Grant's original position, and check the sniper's positions. You'll know you've hit it when a notification comes up informing you that there was a recent unauthorized user. The sniper on the terminal building is harmless: pay no attention to him. Due to an oversight by the developers, there is no sniper on the ridge to the north, nor is there any way to make your way onto the ridge. The sniper on the generator tower behind the president's podium is also innocuous, though the tower isn't. Once you've hacked the computer, make your way to Ranger Grant and tell him you're prepared for the president's visit. The sequence will begin, and you'll have to head straight to the tower. Go up the ladder on the side, and activate the radio. An unnamed soldier will come on and ask what you're doing on the radio. Choose the witty retort, and then sit through the president's speech until the Legion sniper appears and attempts to shoot the president. Since you spoke on the radio, you don't even have to fight the person, as the other Ranger on the tower will kill him, and the Legion sniper won't shoot back. After his death, use the radio to tell Grant about the sniper. After that, you'll win at the quest. ALTERNATIVELY TO THAT, you could use a sniper rifle of some sort (best shot is the anti-materiel rifle) to kill the engineer on the helipad after performing the aforementioned steps, but before the sniper shows up. After the engineer's death, the Quest will be done with.
 * You must be on good terms with the NCR for this quest to be made available. If you are not, it will fail automatically and the quest will skip to Part Seven. Wearing a faction disguise will have the same effect even if you are in good standing with them (the faction must be an enemy of the NCR (for example the Brotherhood of Steel or Caesar's Legion).
 * Some armor causes factions' entries to be removed from the "Reputation" listing on your Pip-Boy. If the particular armor that you are wearing while you report back to Mr. House after completing "The House Always Wins V" results in the NCR's entry being removed, you will automatically fail part VI.

Journal entries
Part VI