• Mr. Trump asserted on Sunday that there were secret, undisclosed elements to his new immigration agreement with Mexico, as he sought to deflect criticism that he achieved less than he had claimed.
• The Mexico tariff story offered a case study in Mr. Trump’s approach: When the goal seems frustratingly out of reach, threaten drastic action, set a deadline, demand concessions, cut a deal — real or imagined — avert the dire outcome and declare victory.
• The Democratic 2020 field includes one of the youngest presidential candidates in modern history (Pete Buttigieg, 37) and the oldest one (Bernie Sanders, 77). The party has to decide whether it is time for generational change.
• The Democratic Party gathering in Iowa on Sunday, the state’s first major event of the 2020 election cycle, had the atmosphere of a circus — one with a very, very big tent.
• Joe Biden retains a lead among likely Iowa caucusgoers, but both he and Senator Bernie Sanders have lost ground over the past three months, while Senator Elizabeth Warren and Pete Buttigieg have made clear gains.