Senator Joni Ernst, Republican of Iowa, will not seek re-election next year, according to two people familiar with her plans, creating an opening that could lead to a competitive race in a key state next year as the two parties battle for control of the Senate.

Ms. Ernst plans to make a formal announcement next week, one of the people said, speaking on the condition of anonymity to discuss private deliberations. A spokeswoman for Ms. Ernst did not immediately respond to a request for comment.

Her departure could give Democrats a prime opportunity to flip a seat in the closely divided Senate, though they face headwinds in the Republican-dominated state and the party’s path to winning back the majority remains difficult.

Ms. Ernst’s decision not to run for a third term, which was reported earlier by CBS News, had been rumored for months, as she has had to fend off threats of a more conservative primary challenger and a falling approval rating that showed less than 40 percent of Iowans approved of her job performance.

Ms. Ernst, who was first elected in 2014 and rose to become one of the highest-ranking Republican women in politics, joins a growing list of senators in her party who have opted not to seek re-election next year after being targeted by President Trump or his supporters. Senators Thom Tillis of North Carolina and Mitch McConnell of Kentucky both are retiring. Senator Tommy Tuberville of Alabama, a close ally of Mr. Trump, is running for governor.

Ms. Ernst appeared to have a potential re-election campaign in mind at the beginning of the year when she caved to political pressure from the right and voted to confirm Pete Hegseth as defense secretary.