Tammy Murphy, New Jersey’s first lady, has ended her run for a U.S. Senate seat now held by the state’s embattled senior senator, Robert Menendez, she announced on Sunday in a video posted to social media.

Ms. Murphy said that she had concluded that continuing to compete in the Democratic primary against Representative Andy Kim, a third-term congressman from South Jersey, would mean waging a “very divisive and negative campaign.”

She was unwilling to do that, she said, and instead decided to suspend her campaign, effective immediately.

“With Donald Trump on the ballot and so much at stake for our the nation, I will not in good conscience waste resources tearing down a fellow Democrat,” she said.

An aide said that Ms. Murphy, the wife of Gov. Philip D. Murphy, had held a meeting with county Democratic Party leaders at 2 p.m. on Sunday before making a final decision and notifying her campaign staff members.

Ms. Murphy, 58, entered the race in November and was instantly endorsed by a coalition of influential party leaders, including many whose livelihoods were dependent on the governor, who has nearly two years left in his term. This led to virtually nonstop claims of nepotism by critics, who argued that the governor and Ms. Murphy, a first-time candidate with limited experience, were exploiting New Jersey’s entrenched system of boss politics.

Mr. Kim, 41, jumped into the race in September, a day after Mr. Menendez was charged with accepting bribes of gold, cash and a Mercedes-Benz in exchange for his political influence. Since then, Mr. Kim has successfully yoked Ms. Murphy’s campaign to what he called the same style of “broken politics” that nurtured and protected Mr. Menendez for decades.

The first independent poll of the race showed Ms. Murphy trailing Mr. Kim by 12 percentage points. Other surveys conducted by campaigns and political organizations showed her even farther behind.

Then, in the midst of the high-stakes primary contest, Mr. Kim filed a lawsuit that directly challenged an essential component of the state’s electoral system — a ballot structure designed to benefit the favored candidates of local political leaders.

Last week, Mr. Kim spent more than an hour testifying in court after asking a federal judge to force the state to redesign the ballot before the June 4 primary.

The judge, Zahid N. Quraishi of U.S. District Court for the District of New Jersey, is expected to rule on Mr. Kim’s request in the coming weeks.