DENVER (AP) — In a Denver co-working space between a brewpub and a shop serving the Hawaiian raw fish salad called poke, independent candidates and possible contenders for U.S. Senate and governorships recently plotted how to pry loose Republicans' and Democrats' grip on U.S. politics.

Terry Hayes, Maine's treasurer and a gubernatorial candidate, left the Democratic Party over its domination by teachers' unions. Craig O'Dear, a Kansas City lawyer exploring a challenge to Democratic Sen. Claire McCaskill of Missouri, was active in the GOP but became disillusioned with both parties. Bob Krist de...