Write-ins seem like a quaint, outdated idea in today's U. S. mega-elections, but has it always been this way? If a write-in candidate were to be elected, is there a system in place in the event that they were to decline the position?
First off, let me explain what exactly a write-in candidate exactly is in current election formats in the United States.
There are 50 states, and each state oversees their own election laws, including for party primaries and for presidential general election. Every candidate would always prefer for their name to be printed on the official ballots on every state. But, that requires gathering enough signatures of voters registered in-state to appear on the november ballot (number varies by state) and submitting those signatures before the deadline (varies by state) and paying any required fees (amounts vary by state). Having your name printed on the ballot as a candidate is called "ballot access".
This is a tremendous undertaking. In the Democratic and Republican parties, candidates who win their party nominations are virtually guaranteed ballot access. Nominees for the Libertarian party have enjoyed strong ballot access. Presidential candidates had ballot access to 50 states in 2000, 48 states in 2004, 45 states in 2008, 47 states in 2012, and 50 states in 2016 and 2020. Since 2000, the Green Party has had the next strongest access to ballot. In 2000 Ralph Nader had ballot access in 45 43 states, and ran as a write-in candidate in 4 states. In subsequent elections the Green Party has managed ballot access in roughly 35-45 states and has run write-in campaigns for nominee in most (but not all!) remaining states.
Write-in campaigns are a remedy of last resort for independent or minor party candidates who can't get enough signatures before the deadline to have their name printed on the ballot. But, there are still requirements to be considered a write-in candidate in 32 states, its just that the deadline to submit signatures and register as a candidate are later. 8 states do not permit write-in candidates for president, if folks write a candidates name on ballots in those states that vote will not be counted. 9 states have no registration requirement for write-in candidates.
Who Received the Most-ever write-in votes (by percentage or total) and why?
There have been several third-party candidates who managed to gather substantial portions of the popular vote (between 5% and 16%). Teddy Roosevelt in 1912, George Wallace in 1968, John B. Anderson in 1980, Ross Perot in 1992 and 1996.
However, every one of those candidates were able to get ballot access in all 50 states. John B Anderson is notable as the first independent candidate to manage ballot access in all 50 states, and he filed a lot of litigation in 1980 that served to make it much easier for independent and minor party candidates to appear on the ballot. Ross Perot also appeared on all 50 states ballots as an independent candidate in 1992, before founding the Reform party and running as Reform candidate in 1996.
To answer your question: As far as I can tell, that would be Evan McMullen who received 221,000 write in votes in 2016. He was on the ballot in 11 states, and a write-in candidate in 31 states. He announced his candidacy too late to realistically appear on the ballot on many states, which is why he became a write-in in so many states.
I think the second-place finisher would be Eugene McCarthy in 1976 who received 58,400 write in votes in California and 600 write in votes in Wyoming.
Third place would be Ralph Nader in 2000 who received a total of 48,880 write-in votes between Georgia, Indiana, Wyoming and Idaho.
Was any write-in candidate ever a significant electoral threat or a decisive factor in the election's outcome?
It has long been argued that Ralph Nader served as a "spoiler candidate" in the extremely-close 2000 election. However, in the states with the closest electoral margins, Nader appeared on the ballot. In states where he received write-in votes, the results were not close.
Ditto, Eugene McCarthy's write-in vote tally was less than half of the margin of victory for Ford in California in 1976.
I won't comment on the 2016 election because of the 20 year rule.
edit:syntax
edit 2: correcting misinformation about write-in registration requirements.