Yes, it happened in the 1972 election when George McGovern replaced his running mate late in the campaign against Nixon.
Yes, there is no mention in the constitution to restrict this. There are age and citizenship requirements but no former job requirements.
yes if he is proven not to be an American citizen, If he is found to be corrupt or has been involved in anti American acts.
(why then is Obama in the White House?)
A candidate can drop from the race at any time.
Barack Obama served as US Senator from Illinois prior to becoming a candidate in the 2008 Presidential election.
Hubert Humphrey was the incumbent VP (and presidential candidate ) that Agnew replaced as VP.
In order to run for president in November, a candidate must first be nominated by his party.
when the party's presidential candidate has been chosen but before the election takes place
In the U.S. presidential election of 1872, Horace Greeley, the Democratic Party Presidential Nominee, died 24 days after the casting of the popular votes and 17 days before the casting of the electoral votes. If he had survived the election, based on the November election results, even with 100% of the votes from each of the states he won he would have gotten a total of only 18.7% of the votes.
It usually happens during the summer of the election year, sometime before the party's national convention.
In the US the elections are called primaries as they are the first step to the election of the final candidate. In the presidential elections Barack Obama first had to beat Hillary Clinton among others before facing John McCain who had beaten other candidates from the Republican party.
right before election day.
of course they can
That has always been false; there was not a U. S. Presidential election in 2002. The most recent U. S. Presidential election before 2012 was in 2008.
Primary elections and caucuses and the Democratic and Republican Presidential conventions