Sen. Bob Corker (R-Tenn.) announced he won't seek reelection in 2018 on Sept. 26, joining four other Republican members of Congress who say they won't seek another term. (Sarah Parnass/The Washington Post)

Sen. Bob Corker (R-Tenn.) announced Tuesday that he will not seek reelection next year, another blow to the Republican establishment on the same day the latest GOP effort to revamp the Affordable Care Act failed.

Corker and other Republican leaders in Congress have come under fire from President Trump and his supporters for not delivering in the early days of the administration.

Once considered an ally of Trump’s national security team, Corker traded insults with the president during the August break amid chatter that staunch conservatives would mount a primary challenge to the Senate Foreign Relations Committee chairman.

Corker’s retirement will touch off what is likely to be a highly contested, ideologically driven primary. It also creates a vacuum among Senate Republicans for leaders on national security issues. For now, Corker isn’t planning on getting involved in either contest.

“After much thought, consideration and family discussion over the past year, Elizabeth and I have decided that I will leave the United States Senate when my term expires at the end of 2018,” the Chattanooga Republican said in a statement.

Corker comes from Tennessee’s long tradition of establishment Republican figures who came to the Senate with ambitions that went beyond the state’s expansive borders. Two of the past five Senate GOP leaders have been from Tennessee, while Corker and Sen. Lamar Alexander (R-Tenn.) are two of the most powerful committee chairmen.

Corker acknowledged that his clout led him to consider breaking his pledge, initially made during his 2006 race, to serve only two terms. “As we have gained influence, that decision has become more difficult. But I have always been drawn to the citizen legislator model,” he said in his statement.

But the establishment wing of the Republican Party has been under assault since the tea party movement took hold seven years ago, and even more so in the Trump era, when mild-mannered dealmakers have fallen out of favor with conservative voters who increasingly prefer angry confrontations over ideological outcomes.

Corker faced that dynamic back home over the August congressional break, when he questioned Trump’s stability after the president’s response to the violence in Charlottesville. “He has not demonstrated that he understands what has made this nation great,” he told local reporters.

That prompted taunting tweets from Trump, who said that Corker was “constantly asking me” whether to seek reelection.

Corker wrestled with the decision of whether to run for months, he said, finally coming to peace with the idea of retiring in late August, while at an event in Clarksville. He set a final deadline of Tuesday at noon to make his decision, he said, whatever happened with the health-care vote.

“I came as a citizen legislator, I did, and hopefully I provided some entertainment for you all by being a person who’s not thinking about reelection,” Corker said during an interview in his office. “That’s the way I came, and I want to depart on that same basis.”

Corker revealed his plans to retire just hours after Senate Majority Leader Mitch McConnell (R-Ky.) announced that he would not hold a vote on the latest bill to repeal the Affordable Care Act because it was destined to fail amid defections from GOP moderates.

Corker’s announcement also came just hours before polls closed in the Republican primary runoff in Alabama, in which appointed Sen. Luther Strange lost to Roy Moore, a former state Supreme Court justice. Moore’s insurgent campaign emboldened the sort of anti-establishment figures who have made McConnell a target of enmity and who were searching for a primary challenge to Corker.

Corker almost retired in 2012, but he was coaxed into running again when it became clear that he would be the top Republican on the Foreign Relations Committee.

There is a growing fear among Senate Republicans that other incumbents will retire or face heated primary challenges next year.

Next year’s Tennessee Senate race will give hard-line conservatives their best chance yet to break through in a state where they have failed in recent primaries for Senate and governor — one of the last holdouts against the tea party wave that has swept other Southern states.

Rep. Marsha Blackburn (R-Tenn.) might try to fill the conservative lane. Former representative Stephen Fincher (R), who retired last year with $2.4 million remaining in his campaign account, could try to run as a tea party hero — he was the first non-Democrat to win his western Tennessee seat since Davy Crockett — but with establishment help as someone who worked well with Alexander and Corker.

Some will look to Peyton Manning, the Super Bowl-winning quarterback who went to the University of Tennessee and retired in early 2016 from professional football. Manning is close to Corker, who brought him to a congressional GOP retreat this year.

Corker’s departure will be felt perhaps most acutely in the area of foreign relations, where the Tennessean not only served as his party’s top voice in the Senate but has for years been celebrated as one of the GOP’s best bipartisan dealmakers.

He established his chops early in his Senate career when lawmakers ratified the New START treaty, a strategic arms-control pact that regulates the size of the U.S. and Russian nuclear stockpiles. Corker was one of the key GOP players who negotiated changes that made it possible to bring more conservative votes on board.

As chairman of the Senate Foreign Relations Committee, Corker also tackled nuclear security, joining with ranking Democrat Benjamin L. Cardin (Md.) to design a bill that gave Congress an opportunity to weigh in on a multilateral deal to rein in Iran’s nuclear ambitions before it could go into effect.

More recently, Corker has been the chief go-between for the White House and Congress when it comes to whether the president will certify Iran’s compliance with the deal next month — a decision that could kick off a brutal battle on Capitol Hill over whether to reimpose nuclear sanctions against Tehran.

Corker has fallen into the role of liaison between the Trump administration and the Senate on other matters as well, including sanctions, nuclear threats and foreign wars. He speaks with Secretary of State Rex Tillerson constantly and has frequent contact with the president.

At times, that has meant Corker has been the one holding the Senate back from tackling issues popular with the rank and file. For months, for example, he staved off a burgeoning effort to increase sanctions against Russia — he later explained that he intended to give Tillerson a chance to make a deal with Moscow to improve the bleak war in Syria.