Senator Tina Smith of Minnesota woke early Saturday morning to a nightmare. Her friend, State Representative Melissa Hortman, had been assassinated in her home, killed alongside her husband.

Friends since 2011, when Ms. Smith worked as chief of staff for former Gov. Mark Dayton, the two women had been together the night before at an annual Democratic Party fund-raising dinner.

Ms. Smith soon learned from law enforcement officials that she, too, had been on the suspect’s target list, which included dozens of Democrats. Even for an official who has faced threats for years, both as a politician and earlier in her career as an employee of Planned Parenthood, the killings were both heartbreaking and chilling. Senator Chuck Schumer, the minority leader, requested extra security for Ms. Smith and her fellow Minnesota Democrat, Senator Amy Klobuchar.

In an interview, Ms. Smith attributed what she said was a political environment awash in violent threats and actions to a toxic atmosphere created by President Trump.

“The uptick in political violence and people being willing to use violence as a means to achieve their political ends,” she said. “I believe that it is fueled by the president’s willingness to use violent rhetoric and to cheer on the people that are doing violence on his behalf.”

She returned to the Senate on Monday to increased security concerns and what she saw as cruel treatment from one of her fellow senators, Mike Lee, a Republican from Utah.