Mix of Republicans not turning out and leftists being energized from their hatred of Trump, Republicans are now the party of the lower propensity voters which gives them a bigger ceiling in general elections, but not for special elections. When Trump was out of the picture for most of 2021 (minus January 2021 when he tanked Republicans like a retard), Republicans did well. Then when he came back in 2022 and Roe happened, Democrats were energized again. It'll be tough for Republicans, but they do show up in general elections.
Long term, I think Republicans will be in a good spot because Trump is probably the most flawed candidate ever to win; every future Republican knows that they need the America First vote to have a chance to win a primary and general election, so strict immigration is non-negotiable. That combined with the future nominee being a much more eloquent talker, and better debater, whatever Democrat is propped up is going to have to defend their BS party positions.