This is how the United States Constitution states the President and Vice-President is to be chosen. The Twelfth Amendment changed the process by having President and Vice-President elected together. Notice, nowhere does the Constitution state how candidates will be placed on the ballot.
The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice President, chosen for the same Term, be elected, as follows
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.
The Electors shall meet in their respective States, and vote by Ballot for two Persons, of whom one at least shall not be an Inhabitant of the same State with themselves. And they shall make a List of all the Persons voted for, and of the Number of Votes for each; which List they shall sign and certify, and transmit sealed to the Seat of the Government of the United States, directed to the President of the Senate. The President of the Senate shall, in the Presence of the Senate and House of Representatives, open all the Certificates, and the Votes shall then be counted. The Person having the greatest Number of Votes shall be the President, if such Number be a Majority of the whole Number of Electors appointed; and if there be more than one who have such Majority, and have an equal Number of Votes, then the House of Representatives shall immediately chuse by Ballot one of them for President; and if no Person have a Majority, then from the five highest on the List the said House shall in like Manner chuse the President. But in chusing the President, the Votes shall be taken by States, the Representation from each State having one Vote; A quorum for this Purpose shall consist of a Member or Members from two thirds of the States, and a Majority of all the States shall be necessary to a Choice. In every Case, after the Choice of the President, the Person having the greatest Number of Votes of the Electors shall be the Vice President. But if there should remain two or more who have equal Votes, the Senate shall chuse from them by Ballot the Vice President.
The Twelfth Amendment said the President and Vice President would be voted for as one.
The electors shall meet in their respective states and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves; they shall name in their ballots the person voted for as President, and in distinct ballots the person voted for as Vice-President, and they shall make distinct lists of all persons voted for as President, and of all persons voted for as Vice-President, and of the number of votes for each, which lists they shall sign and certify, and transmit sealed to the seat of the government of the United States, directed to the President of the Senate;--The President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted;--the person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of electors appointed; and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice. And if the House of Representatives shall not choose a President whenever the right of choice shall devolve upon them, before the fourth day of March next following, then the Vice-President shall act as President, as in the case of the death or other constitutional disability of the President. The person having the greatest number of votes as Vice-President, shall be the Vice-President, if such number be a majority of the whole number of electors appointed, and if no person have a majority, then from the two highest numbers on the list, the Senate shall choose the Vice-President; a quorum for the purpose shall consist of two-thirds of the whole number of Senators, and a majority of the whole number shall be necessary to a choice. But no person constitutionally ineligible to the office of President shall be eligible to that of Vice-President of the United States.
Again, no place does the Constitution provide for how these candidates for President and Vice President are to be chosen. In the case of the first election, the overwhelming consensus was that George Washington would be elected. The electors gave him the majority of the votes and John Adams came in second so he was Vice President.
It was with the advent of political parties that different candidates were put forth. In the fourth election Thomas Jefferson defeated John Adams and the political party structure was in full force. It was not until 1910 that voters had a direct say in who the delegates would be that attended the national conventions and how they were directed to vote. The maverick state was Oregon. The primary system gained popularity and became the primary manner in how the political parties selected their candidates.
When people try to tell you the people have a constitutional right to choose the candidates, tell them they are wrong. Anybody can form a political party. Once that political party is formed, that party can nominate their candidate in any way they wish. The political party or the candidate is responsible for meeting the state requirements of being placed on the ballot or they can choose to run a massive write in campaign.
The American people have no constitutional right to select presidential and vice-presidential candidates. They do have a constitutional right in selecting the electors to represent them in the Electoral College. Do not say that we the American people have a constitutional right to determine who the candidate for president and vice-president will be of any specific party. If you as an individual do not agree with the way certain parties chooses their candidates, change parties or start your own party. You can then nominate your candidate any way you want.