"Proximity search" may help with this, but unfortunately most free web services do not feature it (Google is either exact phrase in double quotes; or AND is implied by a space - the words can be pages apart in a very long document, and totally unrelated. Google DOES use proximity as part of its relevance ranking, but you can't "force" a given limit as to how far apart the words occur).

Even if you are not a student/staff with remote login username and password, almost all public universities will give you free guest access to databases and e-journals on site. JSTOR, a very scholarly full text database, gets 13 hits for: "effective manifestos"~10; and 42 for: "influence manifestos"~10. JSTOR has different "packages", so these results may vary greatly at different universities and colleges.

Academic Search Premier unlike JSTOR, does not default to full text search. It gets 73 articles (34 in "Scholarly/peer reviewed" journals) for the default title, journal title, abstract, and subject search for: manifestos and effect* and political. If you "select a field" TX-All Text, you will certainly want to use its proximity search. For example, TX manifestos n20 effect* n20 political gets 53 hits in Scholarly journals, where the words must be within 20 words of each other (including vertical distance in the text), at least in one place in each article.

It is certainly possible to find scholarly material on the web - especially in Google Books. But you will need to try very many word combinations: manifestos "effective change agents" site:edu gets about 12 unique (of "about 125") sites; "influence of manifestos" gets only 5; manifestos "significant factors" "political change" site:edu gets 76 (of "about 234"). These mostly will have the words and phrases very far apart, and often unrelated.