WASHINGTON — Senator John Thune of South Dakota, the second-ranking Senate Republican, announced on Saturday that he would seek re-election, after an aggressive lobbying campaign by colleagues prompted him to put aside concerns about the future of his party and pursue a fourth term.

“I’m asking South Dakotans for the opportunity to continue serving them in the U.S. Senate,” Mr. Thune said in a statement, adding that he could deliver for his state.

“I am uniquely positioned to get that job done,” he said.

The South Dakotan, who turned 61 on Friday, had recently told associates that he was considering retirement, complaining about the strain of congressional service and privately expressing concern about former President Donald J. Trump’s continuing grip on the Republican Party.

But by seeking re-election in a heavily conservative state, Mr. Thune is well positioned to win again and potentially succeed Senator Mitch McConnell of Kentucky, the minority leader, as the chamber’s top Republican.

A host of Senate Republicans leaned on Mr. Thune in recent weeks to run again, but Mr. McConnell was especially aggressive and met privately with him this past week.