WASHINGTON — The Senate's top five Republican leaders have cosponsored legislation to extend until 2017 the Obamacare insurance subsidies that may be struck down by the Supreme Court this summer. The legislation, offered by Sen. Ron Johnson (R-WI), one of the most politically vulnerable Senate incumbents in 2016, would maintain the federal HealthCare.gov subsidies at stake in King v. Burwell through the end of August 2017. The bill was unveiled this week with 29 other cosponsors, including Senate Majority Leader Mitch McConnell (R-KY) and his four top deputies, Sen. John Cornyn (R-TX), John Thune (R-SD), John Barrasso (R-WY) and Roy...