Then there’s the Rossi-Gregoire race in Washington in 2004 and of course, famously, Coleman-Franken.
In 1996 Woody Jenkins in Louisiana has his senate seat stolen by fraudulent votes in New Orleans. Woody refused to concede ever and got tons of volunteers to help investigate. They found a lot of problems, investigated for long time, congress held hearings, but eventually let Mary Landrieu win stay. Dems basically argued that yes. there were irregularities, but that is normal. Just muddied the water enough to sort of claim there was not enough proof, and yes, they were some fraudulent votes, but maybe not enough to overturn the election. Just talked bull... forever, until Reps gave up. This is probably the closest to Trump-Biden scenario unraveling.
And remember, Ron DeSantis and Rick Scott almost had their races stolen out from under them by Passed-Out-On-The-Bathroom-Floor Guy and the Power-Hungry Astronaut, respectively.