Presidential nominee is assassinated?

I believe that it is up to the party to decide. I don't think that it has ever happened before. If someone is nominated and then dies or is incapacitated during their campaign, I believe that party's delegates would choose someone. I am not absolutely sure that it would be either the vice presidential candidate or the person who came in second. I do know that it is NOT automatically the person who came in second. I am pretty sure that the party can decide, as it sometimes happens, that for one reason or another a Senate candidate who has been nominated drops out, (it happened when Barack Obama won and the republican candidate had to be replaced) and then the party's apparatus chooses. My guess, and I'm sorry that I cannot be 100% sure of this, is that the rules are different for each individual political party and vary depending on the particular circumstances as well.

I was curious about this too. It took me less than five minutes on google to find answers (at least with regards to the two major US political parties).
According to Rule 9 of the Republican party (http://www.gop.com/About/Rules6-10.htm) only the Republican National Committee is authorized to fill "any and all vacancies which may occur by reason of death, declination, or otherwise" for the presidential candidate. The committee re-assembles. According to rule 8-B, they must give five days notice for this meeting. Committee members "representing any state shall be entitled to cast the same number of votes as said state was entitled to cast at the national convention." They continue until a new candidate is chosen with a majority of votes.
Democrats have a similar process (http://www.democrats.org/pdfs/charter.pdf). Article Two, Section 7(c) of the Democratic Party rules stipulate that "a special meeting to fill a vacancy on the National ticket shall be held on the call of the Chairperson, who shall set the date for such meeting in accordance with the procedural rules provided for in Article Two, Section 8(d) of these Bylaws." Article Three, Section 1(c) of the rules state only the Democratic National COmmittee may fill vacancies in the nominations for president and vice-president.

what happens if the elected president dies before he is officiallly inaugurated?  does the elected VP move up to Pres?

  He has more threats against his life and is costing the American Tax payers more money than any president in history.  He must walk a careful walk and talk a careful talk or he will not only bring more of the same upon himself but he will most assuredly bring this Nation to Civil Unrest or Civil War. 

  The American people will not tolerate his thoughts on guns, they will not tolerate his selling Israel down the tubes, they will not tolerate a Civilian Army, you kidding me, he will bring unrest like never before if he even tries.  They will not tolerate being forced to serve the public need.  They will not tolerate too many more taxes.  They will not tolerate any comments from his wife that so much as suggests any anti American sentiment.  They will not tolerate his taking from one to make the other equal, the other has the same opportunities and more.  They will not tolerate Amnesty.  This is a bleeding open wound.  It is far from healed.  The answer is no for many reasons, we are in economically pressed times, we don't need people who need to be subsidized we need people coming into this country of substance.  And if he pushes Spainish down our throats as a must in schools he will be putting himself in jeopardy.

There is so much more than meets the eye.  He is well aware 57000 Americans did not vote for him and we are the cream of the crop while what voted for him was the cream of the crap.  Don't think he does not know this.  His voters can not uphold this country, if the 57000 who did not vote, make the moves I have made concerning my investments and 401k he is in deep camel dung.  He knows it.  He will do everything in his power to win the likes of me over.  But we are the intelligencia and it won't happen.  We know him, understand him, and we will fight him on every move he makes.

His presidencey is a sticky wicket and he knows it.  He knows what he has said in his books, speeches, he knows who his ancestors are and he knows you can fool some of the people some of the time but there are 57000 unmoved, unrelenting, unforgiving with memories that shame an elephant.

This is not going to be pretty.  However, I surely hope some looney tunes out there is not considering something that is absolutely unacceptable to any of our thoughts or palates.  In this situation my prayers are for him.  I don't want anything to happen to him unsavory.  This is the year 2008 we must not be savages.  If we don't agree with him we must let him know in ways acceptable to the times.  Please nobody wish him bodily harm.

Simonstudio you and I know the truth.  "The witches stirred the brew and they howled like banshies under the full moon as all the Universe assisted in their efforts and said yes, so let what be, be and so it was."