Why is red for Republicans and blue for Democrats?

Red is for Republicans and blue is for Democrats, an association that only became cemented during the 2000 election.

How Blue States & Red States Use COVID Relief Funds Differently to

なぜアメリカでは「共和党が赤、民主党が青」で分けられているのか？ - GIGAZINE

The Rise of Negative Partisanship and How it Drives Voters

You'll Never Guess Why Republicans Are Red, Democrats Blue - ABC News

Our Maps Shouldn't Lie About Our Votes - Sightline Institute

Republicans Are “Clustered” By County, Democrats Are “Clustered

Political And Electoral Instability

How Voters Turned Virginia From Deep Red to Solid Blue - The New

How Red or Blue are Michigan's Political Maps and Election