Within the politics stream of the Multiple Streams Approach there are national mood, balance of interests [presumably akin to the relative power of coalitions in the policy subsystem?] and party ideology.
“the aggregate orientation of the political parties within relevant institutions”
(Jones et al. 2016:16)
Jones, M. Peterson, H., Pierce, J. Herweg, N., Bernal, A., Lambert Raney, H. and Zahariadas, N. 2016. A River Runs Through It: A Multiple Streams Meta-Review. Policy Studies Journal, Vol. 44, pp.13-36.
This can be shifted by strategic donations and the rise of a new ideology (e.g. the conservative parties of Australia, US and UK all changed, during the 70s and 80s, from a sort of paternalistic ‘conservatism’ to determined neo-liberalism.
See also the rise of the “Tea Party” and its influence on the Republican Party.