"We are reminded, however, that 'for everything there is a season,' and I believe that the time has come for a change," he added. "Therefore, I will not be a candidate for reelection in the 2020 election."

Thornberry's announcement caps off months of speculation about the Texas Republican's political future.

The 13-term lawmaker’s retirement adds to a growing list of Republican departures, especially in the Lone Star State. He's the sixth Texas GOP member of Congress to retire so far ahead of the 2020 elections.

Already, three other House Armed Services Republicans — Reps. Rob Bishop of Utah, Mike Conaway of Texas and Paul Mitchell of Michigan — have opted to retire.

Another Texan, Rep. Will Hurd — the only black House Republican — made waves when he announced his decision not to seek reelection after just three terms.

Thornberry's congressional district in the Texas panhandle, home to the Pantex plant that handles nuclear weapons, is among the most conservative in the country and is almost certain to stay in Republican hands in the 2020 election.

Thornberry is locked in negotiations with other House and Senate Armed Services leaders over annual defense policy spending that are likely to extend well into the fall.

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In nearly a quarter century on Capitol Hill, Thornberry was widely respected for his national security acumen by lawmakers in both parties.

His Democratic counterpart, House Armed Services Chairman Adam Smith of Washington state, praised Thornberry's work to reform the Pentagon and called his retirement "a loss for the Congress."

"I have known Mac since I first came to Congress in 1997 and have always been impressed by his intelligence and tenacity. Since then we have worked together on all manner of national security issues as members of the House Armed Services Committee," Smith said in a statement. "Together, we have passed smart reforms that give our men and women in uniform the resources they need to make our country safer."

Thornberry's retirement announcement will likely trigger a rush to replace him on the committee. He was term-limited from his committee perch in 2021 as the top Armed Services Republican — or chairman if the GOP retakes the House — in the next Congress.

Republican Reps. Mike Turner of Ohio and Mike Rogers of Alabama have long been seen as likely candidates to succeed Thornberry. Other senior Republicans on the committee, such as Rep. Joe Wilson of South Carolina, could also seek the post.

Thornberry took over as Armed Services chairman in 2015, becoming the first Texan to lead the committee.

In four years wielding the gavel, he advocated — and won — major increases in the defense budget that pushed military spending well past $700 billion. Total national defense spending for the 2020 fiscal year is set to top out at $738 billion, a historic high in military spending outside the height of the Iraq and Afghanistan wars.

His chairmanship, which coincided with other senior Texans chairing national security committees, arguably marked the pinnacle for the state's influence on defense spending and security issues.

Thornberry criticized strict spending caps that have limited the defense budget, though he voted for the 2011 Budget Control Act that put those limits in place to rein in a growing deficit. And he tied a series of deadly aviation accidents, ship collisions and other readiness issues to a lack of steady, predictable funding for the military.

As Armed Services chairman, reforming the Pentagon’s arcane weapons-buying practices became one of Thornberry’s signature efforts. He criticized the acquisition system as far too slow and rigid to field weapons quickly enough to meet rapidly evolving threats. He also backed provisions aimed at streamlining the acquisitions process in each defense policy bill he spearheaded.

Additionally, he sought to slash the Pentagon’s support and administrative agencies, commonly known as the Fourth Estate — a move that drew resistance from Republicans and Democrats who sought to protect defense agencies in their districts.

When Democrats retook the House majority in the 2018 midterm wave, Thornberry became the panel’s ranking member. He led all House Republicans in opposing defense policy legislation written by Democrats in July, arguing the bill would have jeopardized military readiness by endorsing a lower level of defense spending, among other contentious provisions.

Thornberry was elected to Congress as part of the 1994 Republican Revolution that swept the GOP into the House majority for the first time in four decades. He's one of only two Republicans left in the House who was elected in the 1994 midterm wave; the other is Rep. Steve Chabot of Ohio.

Prior to running for office, Thornberry worked on Capitol Hill as an aide to Texas Republican Reps. Tom Loeffler and Larry Combest and in the State Department during the Reagan administration.

Thornberry also hails from a longtime ranching family. And when asked on several occasions if he'd run for open Republican leadership posts, his response was usually terse.

"I'd rather be a vegetarian," he said.