The modern Republican Party’s grand

The modern Republican Party’s grand, radical agenda lies in ruins — but the party doesn’t know how to deal with that failure, and it retains enough power to do immense damage as it strikes out in frustration.

  1. What does the bold part suggest?
  2. Does it modify “radical agenda”?

Only in that the adjective ‘grand’ is used as an adjective along with ‘radical’ to describe the agenda.

Then is the comma wrong here?

No, commas separate adjectives when there is more than one adjective describing the noun.

Does “as” mean “because” here?

No, it means ‘while’.