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.
In order to run for president in November, a candidate must first be nominated by his party.
Yes, and it has happened before. In the 1972 presidential election, Democratic candidate George McGovern had chosen Misssouri Senator Thomas Eagleton as his running mate for vice-president. During the campaign it was learned that Eagleton had undergone electric shock thereapy and had been hospitalized for mental conditions. McGovern, initially supportive of Eagleton, later agreed that he should not remain as the vice-presidential candidate because of these issues. Eagleton dropped out and McGovern changed his vice-presidentail running mate to R. Sargent Shriver.
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
No candidate loses until the ballots are counted. How can you be certain that a candidate is a "losing candidate" before the election is held?