Political parties are weak in the US for two main reasons.
First, they are weak because we have primary elections. In a primary election, candidates only have to appeal to voters. They do not have to keep in the good graces of the actual party establishment.
Second, political parties are weak because of the strength of interest groups. Parties try to bring together big coalitions that can agree over a broad range of issues. This calls for compromises. Interest groups insist on getting their way on specific issues. This works against compromise and makes it harder for party coalitions to stay together.
I would disagree that political parties are weak in our country. There are two main political parties in the United States. If a person is not a member of one of those political parties, it is very hard to get elected to a state or to a national office. This suggests our political parties are strong.
Political parties reflect the will of their members. The party members determine the candidates of the party through the primary and the caucus system. The party members influence the party’s goals that are reflected in the party’s platform. It used to be that party leaders dictated who the candidates would be and what the party would represent. That is no longer the case. Political parties are stronger because of their members.
The strength of political parties can be seen in the gridlock that has occurred at the national level. Candidates and elected officials feel the need to support the goals and candidates of the party or face significant consequences, even if they don’t like the candidates or if they disagree with the goals. This has led to less compromise, which isn’t a good thing, but reflects the influence the political party has.
There are some forces that can weaken the influence of the political parties. For example, outside interest groups can advocate for a particular party’s candidate and not be under the control of the political party. These groups can send messages and run ads that the political party doesn’t endorse. There is very little a political party can do about that.
Overall, it isn’t realistic to say that political parties are weak. The evidence suggests otherwise.