With millions of Americans casting their ballots by mail during the pandemic, The Guardian and ProPublica are tracking the votes in critical states to determine how many are counted, rejected and delayed

This piece is published in partnership with ProPublica, a nonprofit newsroom that investigates abuses of power. Sign up to receive their biggest stories as soon as they’re published.

An unprecedented number of Americans are voting by mail this year to avoid Covid-19 risk. Joe Biden’s supporters have said they are more likely to vote by mail while Donald Trump’s supporters say they are more likely to vote in person. With postal delays, rejected ballots and a dearth of funding, the process isn’t always smooth – ballots can be rejected for multiple reasons, and due to court challenges, election rules are changing even while voting is underway. Meanwhile, Trump and other Republican officials have spent the last months casting doubt on the mail-in voting process, paving the way for legal battles during the vote count.