Why did American political parties get weaker on purpose?

The "smoke-filled room" lost legitimacy by the 1970s. McGovern-Fraser reforms transferred presidential nomination power from delegates to primary voters. Campaign finance reform broke party fundraising. McCain-Feingold further weakened. The result is a party system where the formal party has very little control over who runs under its name. Trump in 2016 and Sanders in 2016/2020 were both candidates the formal party didn't want. Both got close to or won the nomination. This wasn't an accident — it was the *intended* result of half a century of reforms aimed at "democratizing" the party. The unintended consequence is that without strong parties to filter candidates, you get an attention-driven primary, which selects for *being interesting to the most-engaged voters* rather than for general-election viability. If you want robust general-election candidates, you probably want strong parties. The trade-off is paternalism — strong parties choose for you. The 1970s reformers preferred low paternalism. The current populism era is the bill coming due.