The Obama administration appears to be on the verge of a diplomatic breakthrough with Iran after three and a half decades of low-level warfare. Now comes the hard part: convincing Congress. Skeptical lawmakers are gearing up to thwart efforts to lift the pressure on Iran. Senate Foreign Relations ranking member Bob Corker (R-Tenn.) warned this week that he may introduce legislation making it harder for President Obama to loosen existing sanctions, while several others have vowed to slap on new ones. “The United States should negotiate from a position of strength, not weakness,” Sen. Ted Cruz (R-Texas), a possible 2016...