Why only before? There is no right to run for nor to be elected President. What specific injury did McCain suffer? No McCain would not have had standing either, although it is nice to pretend that he would have, if only he'd acted "in time". But alas, he didn't, and so the case is now moot. Somewhat like Article II, Section 1, clause 5 of the Constitution.
Every year, and all across the country, ballot access for specific candidates is challenged by opposition parties or candidates. Those cases are heard on the merits, and sometimes they prevail.
Most commonly, these challenges have nothing to do with the age or citizenship status of the allegedly ineligible candidate. But rather, they usually surround the qualifying petitions the candidate has collected and submitted and specifically whether or not the candidate has indeed presented either the requisite number of signatures, or there's a question with regard to the quality/authenticity of those respective signatures.
Standing and justiciability are never an issue in these cases that involve legitimate opposition parties or candidates.
From Judge Carter's opinion:
P 16-17
"Plaintiffs argue that the injury they suffered was the deprivation of the right to run for office on a fair playing field against only candidates who meet the constitutional requirements to serve as President. Under this theory, the injury is not that of being deprived the chance to win, but being deprived the chance to compete only against “legitimate” candidates. If the Court accepts this concept of injury, then all candidates would have standing to sue the President on the basis that they were all injured by having to compete against him in the national election.
Because the political candidate plaintiffs are the only category of plaintiffs who potentially satisfy the injury-in-fact requirement, the Court will turn to whether the political candidates can satisfy the redressability requirement of the standing analysis and whether the political candidates can further clear the political question and separation of powers hurdles of justiciability."
[...]
"Ultimately, Plaintiffs’ alleged injury is having to respect the authority of a president who does not meet the constitutional requirements to hold office. Therefore, Plaintiffs’ injury would only be redressed by the removal of President Obama from office."
[...]
"In order for Plaintiffs’ alleged injury to be fully addressed, Plaintiffs would have the Court intervene, upheave the results of a national election, declare the President illegitimate, shut down the functioning of the government of the United States, and leave this country defenseless.
P 18
"Because Plaintiffs did not file this action until the day President Obama took office and was sworn in, any action that this Court takes in this matter is not merely against Senator Obama as a political candidate but against President Obama, this country’s sitting president. In this case, the redressability prong of standing is intimately intertwined with and influenced by another justiciability concept–political question and the separation of powers. Any action taken by the Court would necessarily infringe upon, at the very least, the Executive branch because it would involve a declaration regarding the qualifications of the President. Because the redressability analysis must consider what actions the Court may take against a sitting President, separation of powers concerns regarding the appropriate role of the judiciary sit at the forefront of the redressability analysis."
P 20
"Removing the President would not only affect the Executive branch, it may also infringe upon the power of the Legislative branch granted by the Constitution in matters of Presidential impeachment and succession. Defendants argue that the Constitution grants Congress the sole power to remove a president through Article I, Sections 2 and 3, which address impeachment, and the Twenty-Fifth Amendment, which addresses the removal of the president should he or she be unfit to serve.
The non-justiciability of an action on political question grounds is “primarily a function of the separation of powers” and pertains to “the relationship between the judiciary and the coordinate branches of the Federal Government.” Baker v. Carr, 369 U.S. 186, 210, 82 S. Ct. 691 (1962)."
Pp 24-25
"There may very well be a legitimate role for the judiciary to interpret whether the natural born citizen requirement has been satisfied in the case of a presidential candidate who has not already won the election and taken office. However, on the day that President Obama took the presidential oath and was sworn in, he became President of the United States. Any removal of him from the presidency must be accomplished through the Constitution’s mechanisms for the removal of a President, either through impeachment or the succession process set forth in the Twenty-Fifth Amendment. Plaintiffs attempt to subvert this grant of power to Congress by convincing the Court that it should disregard the constitutional procedures in place for the removal of a sitting president. The process for removal of a sitting president–removal for any reason–is within the province of Congress, not the courts."