The STEADFAST Act redirects federal funding away from subsidizing presidential campaign spending and instead uses that money to help states improve their election security systems. Currently, taxpayers can voluntarily contribute $3 to a Presidential Election Campaign Fund on their tax returns, which provides public financing for presidential candidates. This bill eliminates that program entirely and redirects all that funding to a new Election Security Fund. States can apply for grants from this fund to upgrade their voting equipment, improve cybersecurity protections for voting systems, purchase paper ballots, secure electronic poll books, and strengthen physical security for voting equipment storage. To receive funding, states must certify they don't allow noncitizens to vote and must submit plans explaining how they'll use the money. States that already use paper ballots, verify voter citizenship, and require photo ID at polling places get priority for receiving grants. The amount each state receives is based on its registered voter population. Any leftover money from the old presidential campaign fund gets transferred to the new election security fund. States must report back on how they spent the money and post those reports publicly.