In Senate Race Shake-Up, Frustration With Establishment Politics

The stage was set, as it often was in New Jersey politics.

The Democratic machine that had kept Senator Robert Menendez in power had abandoned him, and into the spotlight stepped Tammy Murphy, the state’s first lady, who had decided to run for his seat.

She quickly amassed support from Democratic Party leaders in the state’s largest counties, bestowing an air of inevitability to her candidacy soon after she entered the race in November.

But on Sunday, she abruptly ended her campaign. It was a tacit acknowledgment that she and her husband, Gov. Philip D. Murphy, had grossly miscalculated voter sentiment as they made a play for a coveted seat at a high-stakes moment in history. Ms. Murphy’s flop also reflected intense national frustration with politics as usual — energy that her chief rival successfully tapped.

“The machine overplayed its hand,” said Uyen Khuong, the director of Action Together New Jersey, a nonpartisan volunteer group that advocates voting rights.

“When they tried to shove the candidacy down voters’ throats,” Ms. Khuong said about the first lady’s Senate bid, “they went too far.”

Ms. Murphy’s exit leaves Andy Kim, a 41-year-old congressman from South Jersey, as the odds-on favorite to be elected in November. If successful, he would be the country’s first Korean American senator and one of the Senate’s youngest members.