In the race for City Council’s seven at-large seats, a pair of candidates from the Working Families Party are mounting the strongest third-party campaign in memory, threatening to capture seats that are guaranteed to independents or minority-party candidates under the city’s Home Rule Charter and that have been held by Republicans for 70 years. And in the 10th Councilmanic District in Northeast Philly, a 40-year Republican incumbent is facing a serious challenge from the Democratic nominee, a political newcomer.