Do you have a preferred specialty area of political science that you would like to study ?
Typical areas of specialization at the PhD level for Political Science include:
American Politics, Comparative Politics, Political Methodology, & Political Theory.
Since you are limiting your target schools to 6 universities, I suspect that your list of 6 comes from these 8 universities: Harvard, Princeton, Stanford, UC-Berkeley, Michigan, Yale, Duke, & Columbia. Chicago & NYU may be worth considering as well.
Of course, there are a few dozen great universities that one might consider for a PhD in Political Science.
Without significant (preferably published) research & a higher GRE, my best guess is that you might want to expand your list of targeted schools from these universities:
In addition to the 10 that I listed above, consider:
UNC, Ohio State, Wisconsin, WashUStL, UC-San Diego, UC-Davis, UCLA, MIT, Univ. of Rochester, Cornell, Penn, Northwestern, Texas, Texas A&M, Illinois, Minnesota, Vanderbilt, Indiana, Michigan State, Virginia, Pittsburgh, GWU, Maryland, Penn State, Rice, University of Washington, Iowa, Notre Dame, Brown, Univ. of Colorado at Boulder, Emory, Georgetown, FSU, & SUNY at Stony Brook.