Maybe I'm misunderstanding what you mean by "strong" vs "weak", but I think it has a lot more to do with how society is changing.
Our country is more educated and racially diverse than ever. Meanwhile, income inequality is the highest it has been in a long time. Of course that's (among other reasons) going to change the type of people who get elected.
Parties are weak at the state and local level when the party leaders have no ability to select the party's nominees.
Nearly all, maybe all, nominees are determined by primary elections. But who determines who votes in those primaries? Not the parties. The state determines it. Different states have different rules, which in itself doesn't bother me. But some states have open primaries, where anyone can vote in any primary regardless of his/her party affiliation. Some states allow people to change parties right up until a few weeks before the primary elections.
Under conditions like that, how can we hold the parties responsible for the people who have (D) or (R) after their names on the general-election ballot?
Parties are also weak at the national level when they can't determine their nominees, although the mechanics of why that is are different.
I laughed at the Democrats in 2016 because they were actually less "democratic" than the Republicans. Had they been more democratic, Bernie Sanders would have been the nominee. But the Democrats got smart after the George McGovern debacle in 1972 and set up a system where there are a large number of super-delegates--selected by the party--at the nominating convention.
So now I think that the Democrats do it better than the Republicans. If the Republicans had had more super-delegates, maybe Trump wouldn't have gotten the nomination. But the Republicans are more democratic in their selection process, and so Trump was able to successfully hijack the party.
I think both parties, in their own interest, should go one better than the Democrats and make at least half of the delegates to the nominating conventions be super-delegates, chosen by party leaders at local, state, and national level.
I don't think racial diversity and income disparity have much to do with it at all. The "weaker," more democratic party, the GOP, nominated a lifelong-Democrat populist rabble-rouser who did not share the party's positions on many issues.
The Atlantic had a great opinion piece a couple of months ago: "
Too Much Democracy is Bad for Democracy."