Pennsylvania's Supreme Court ordered state officials last week to throw out mail-in ballots submitted without a required inner "secrecy" envelope in November's election, the Philadelphia Inquirer reports.

The state of play: The decision went under the radar alongside the simultaneous decision to extend the time that mail-in ballots could be counted, but Philadelphia's top elections official warned state legislators this week that throwing out so-called "naked ballots" could bring "electoral chaos" to the state and cause "tens of thousands of votes" to be thrown out — potentially tipping the presidential election.

• Pennsylvania requires voters to place their ballots in an unmarked "secrecy" envelope before placing that inside another mailing envelope.
• Historically, only about 5% of Pennsylvanians have voted by mail as the state had required an excuse to vote absentee. This year marks the first time that the state has adopted no-excuse absentee voting, so many will be voting by mail amid the coronavirus pandemic — and using the double-envelope system — for the first time.
• The decision to reject "naked ballots" didn't apply during the primary earlier this year, which was the first use of expanded mail-in voting, so it's unclear how widespread the mistake may be. However, 6.4% of ballots were "naked" during last November's municipal election in Philadelphia, which was conducted under the more restrictive absentee system.

Why it matters: Polls have found that more Democrats than Republicans plan to vote by mail, so thrown-out "naked ballots" are more likely to be cast for Joe Biden. President Trump carried Pennsylvania by just 44,000 votes in 2016.

• "Pennsylvania is so important that our model gives Trump an 84 percent chance of winning the presidency if he carries the state — and it gives Biden a 96 percent chance of winning if Pennsylvania goes blue," FiveThirtyEight's Nathaniel Rakich noted in a recent analysis.
• If tens of thousands of "naked ballots" are rejected by election officials, it could be enough to swing the result in a tipping-point state.

What they're saying: "[R]ecent actions by the Pennsylvania Supreme Court have set Pennsylvania up to be the subject of significant post-election controversy, the likes of which we have not seen since Florida in 2000," Lisa Deeley, the chair of Philadelphia's city commissioners, wrote to state legislators.

• "I hope you consider this letter as me being a canary in the coal mine."

The bottom line: Cook Political Report's Dave Wasserman called the situation "a foreseeable train wreck."

Read Deeley's letter.