Planning to write in Paul Ryan or Bernie Sanders?
It won’t count in most states.

In an election year with two historically unpopular nominees, write-in candidates can be enticing to undecided voters who can’t find someone to get behind, or who are searching for a way to voice their disappointment.

And it’s not just the Bernie Bros: Even high-profile politicians like Republican senators Kelly Ayotte (N.H.) and Rob Portman (Ohio) say they  will write in Donald Trump’s running mate, Mike Pence.

[ How to vote in your state]

But because of state laws restricting ballot access, writing in a candidate is not as simple as it sounds. There are eight states where ballots won’t even have a space for write-ins. In 32 others, write-in candidates have to file with the state prior to the election to be counted as official candidates.

That means that even if a write-in candidate wins the popular vote — an extremely unlikely scenario — their votes won’t be counted if the candidate hasn’t pre-registered. States often throw these write-in votes for unregistered candidates into an “All others” category and don’t tally them up individually.

[ As Clinton builds a lead, write-in campaigns flower and falter]

Which candidates are options in each state?

Every state will have at least three options on its ballot for president: Democrat Hillary Clinton, Republican Donald Trump and Libertarian Gary Johnson.

Green Party candidate Jill Stein will be on the ballot in all but six states, but she is available as a write-in candidate in half of those six.

Independent candidate Evan McMullin is on the ballot in 11 states, including his home state of Utah. If the conservative-leaning candidate wins there, it would be the first time someone other than a major-party nominee would win electoral votes since 1968. Voters can officially write him in at polling places in more than two dozen other states.

For comparison, here are the places where voters can write in anyone — from Bernie Sanders to Mickey Mouse.

To be an official write-in in California, 55 people must sign a petition for a nominee. But that person doesn’t have to consent, which is why Bernie Sanders is an official write-in in California even though he isn’t running for president.

[ Bernie Sanders: ‘This is not the time for a protest vote’]

How many options are there in each state?

Most states have fewer than a half-dozen presidential candidates on the ballot, but write-in options exist in most states.

Getting ballot access can be hard for smaller campaigns with fewer resources. Darrell Castle, the candidate for the Constitution Party, is on the ballot in more than two dozen states. Castle said his campaign has faced ballot access obstacles in California, Texas and Oklahoma. “Those states are virtually impossible for a party like mine because they cost so much money,” Castle said.

Castle isn’t on the ballot in Oklahoma. To be on the ballot there required signatures from at least 3 percent of all votes cast in the state in 2012, or about 40,000 signatures. Castle says this would likely require hiring professionals who often charge a few dollars for each signature.

Roque “Rocky” De La Fuente is running as an independent and is on the ballot in 20 states. As other non-major-party candidates have promised to do, De La Fuente has mounted several legal challenges for ballot access.

Registering as an official write-in is only half the battle

Even in states where a voter can write in anyone, there are still hurdles for a candidate to emerge victorious. Some states and the District of Columbia require post-election paperwork from any winning write-in. In Mississippi, write-ins are considered only in the event of the death, resignation, withdrawal or removal of a candidate already on the ballot.

States also differ in how they standardize write-ins on a ballot. Aside from spelling a candidate's name correctly, some are more lenient with nicknames and initials than others. Massachusetts allows candidates to run sticker campaigns, and Pennsylvania allows stamp campaigns. Some others strictly prohibit these, making it even harder for write-in candidates to get any votes.

Not that people won’t try. In 2012, a man legally changed his name to “Santa Claus” and registered as a write-in candidate in Maryland. In all, 625 people wrote in “Santa Claus” for their presidential vote, the most for any write-in in Maryland that year.

[ Who are you writing in? The overwhelming allure of voting for someone who won’t win.]

According to the Federal Election Commission, nationwide write-in votes have increased more than fivefold since 1984. This is probably an underestimate, because some states only tabulate write-in votes if a race is close.

Write-ins usually account for less than 1 percent of all votes cast. Alaska had the highest share of write-in votes in 2012, at 0.96 percent, two years after the state’s high-profile Senate race was won by a write-in incumbent.

That race was the exception. At current levels, write-in voters alone are not enough to fuel a winning presidential campaign.

But that won’t stop tens of thousands of voters from supporting write-ins in 2016, whether on principle, or as an act of protest of the two major-party candidates.