Sen. Pat Roberts (R-KS), chairman of the Senate Agriculture Committee, announced Friday he will not seek another term in the U.S. Senate.

JUST IN: GOP Sen. Pat Roberts of Kansas announces he won’t seek reelection in 2020. “Never, never in my wildest dreams did I think I would become the longest-serving member of Congress in our state’s history” pic.twitter.com/soZ1BPpLbD — TicToc by Bloomberg (@tictoc) January 4, 2019

Speaking before reporters in Manhattan, Kansas, Roberts said: “I am announcing I will serve the remainder of this term as your senator, fighting for Kansas in these troubled times. However, I will not be a candidate in 2020 for a fifth Senate term.”

“Never, never in my wildest dreams did I think I would become the longest-serving member of Congress in our state’s history,” he added.

Roberts, 82, maneuvered the 2018 farm bill through Congress with support from both parties — a move ultimately embraced by President Donald Trump as a rare bipartisan success. However, some Republican critics argue Roberts’ work across the aisle resulted in a deeply flawed and costly farm bill that represents a betrayal of conservative values. “It’s a slap in the face to Ronald Reagan,” said Robert Rector, senior research fellow at the Heritage Foundation. “It’s a victory for a massive welfare state without work requirements and an out-of-control budget.”

No one had yet announced a primary challenge against Roberts, but conservative activists viewed him as vulnerable from the right. He narrowly survived a primary against tea party candidate Milton Wolf in 2014. Republicans including outgoing Gov. Jeff Colyer (R) and Rep. Roger Marshall (R-KS) are eyeing a race in anticipation that Roberts might not run.

In a December opinion-editorial, The Kansas City Star‘s editorial board called on Roberts to retire and “step aside to allow other qualified candidates to seek the office.”

“[T]he 21st century will demand fresher ideas — not just on farm policy, but on trade, immigration, taxation, spending, defense and other national challenges. Roberts isn’t the senator who will provide those new ideas,” the bluntly stated op-ed read. “This isn’t a matter of the senator’s age, although Roberts will be 84 years old in 2020. It’s much more about Roberts’ four decades of service in the House and Senate, a long tenure that makes it difficult for him to see things in a different way.”

Roberts has served as the senior senator from Kansas since 1997 and was elected in his home state’s First Congressional District to the House of Representatives in 1980.

Sen. Lamar Alexander (R-TN) announced last month that he will not run for re-election in 2020.

The Associated Press contributed to this report.