In the late afternoon of May 22, 2001, an apocalyptic set of thunderstorms rolled across the Mall, with bolts of lightning striking just outside the Capitol dome. Inside the Senate chamber, an even bigger storm was brewing — James M. Jeffords, the venerable patrician senator from Vermont, had informed Democratic and Republican leaders that he was likely switching parties.

Two days later, after 26 years on Capitol Hill, Jeffords would leave the Republican Party to caucus with the Democrats. The move ended a historic five-month run in which the Senate sat deadlocked at 50-50, with Vice President Richard B. Cheney giving Republicans control with his tie-breaking vote. Jeffords, who died Monday at age 80, was handing Thomas A. Daschle (D-S.D.) the title of majority leader and putting the brakes on the domestic agenda of the Bush White House.

“Lord,” Don Nickles (R-Okla.), then the Senate majority whip, shouted as he walked off the floor that Tuesday afternoon, looking out at the crackling skies as if to find a metaphor for the storm inside the Capitol. The next night, as coincidence would have it, the entire Senate gathered for another episode in the now-extinct “Leader’s Lecture” series. The event was one of Gerald R. Ford’s last big speeches at the Capitol.

“A few mistake the clash of ideas for a holy war,” the former president, vice president and House minority leader told the senators. Jeffords was not present for the Ford speech; he was en route to Burlington to drop his bombshell — and echo Ford’s lament.

It was a singular moment when one lawmaker, with one vote, truly bent the arc of politics in a different direction. It also served to highlight the feud between the still-dominant conservative wing and the increasingly marginalized moderate faction of the Republican Party. “Given the changing nature of the national party, it has become a struggle for our leaders to deal with me and for me to deal with them,” Jeffords said in Burlington.

This May 25, 2006 file photo shows Sen. Jim Jeffords, I-Vt. on Capitol Hill in Washington. (Charles Dharapak/AP)

The entire episode, from the point at which voters left the Senate at 50-50 until Jeffords’s switch, serves as a guidepost to what could happen should November’s midterm elections leave the Senate similarly deadlocked.

In May 2001 there was no social media, cable news focused more on car chases than Congress, and throughout that pre-9/11 summer the nation was gripped by shark attacks, the controversy surrounding the true age of a Little League pitcher (Danny Almonte) and Gary Condit’s affair with a slain intern. The most important half hour of political television came every weekday at 4 p.m., when CNN’s then-version of “Inside Politics” would run, with Judy Woodruff as the host.

It was CNN’s congressional reporting team of Jonathan Karl (now ABC News White House correspondent) and Dana Bash (back at the congressional beat for CNN the past few years) that broke the news that Jeffords was leaving the GOP, declaring himself an independent, and would caucus with Democrats. The move followed weeks of feuding between Jeffords and President George W. Bush’s White House.

The switch was months, if not years, in the making, given the moderate style of GOP politics in New England, where the Bush family had its roots, vs. the Texas-style conservatism adopted by the latest generation of Bushes.

On Election Day 2000, Republicans entered with a ­54-46 edge in the Senate, but faced a terrible map — incumbents up for reelection in a number of states in which Vice President Al Gore would defeat Bush: Minnesota, Washington, Delaware and Michigan. All four lost, and after the shakeout in a few other states, including an epic Missouri race in which a dead governor defeated incumbent John Ashcroft (R) and was replaced by his widow, Daschle had reached 50 seats in his caucus and entered into negotiations with Trent Lott (R-Miss.), the Republican leader.

After days of talks, the two leaders emerged with a plan that would leave the chairmen’s gavels in the hands of Republicans, including those of Jeffords, who was chairman of what is now called the Health, Education, Labor and Pensions Committee.

In a recent interview, Lott recounted that his Republican chairmen were so furious with the deal that they nearly forced him into resigning as GOP leader during a tense meeting at the Library of Congress. Finally, Lott recalled, Sen. Pat Roberts (R-Kan.) stood up and rallied his fellow Senate Republicans around the plan as the best they could possibly hope for given the 50-50 split in the chamber.

Democrats began maneuvering to find a Republican who would switch sides and give them full control, with Sen. Harry M. Reid (D-Nev.), then the minority whip, playing a key role. Jeffords was one of three senators targeted, along with Lincoln D. Chafee (R.I.), who was then a Republican but would later follow Jeffords’s lead, first becoming an independent and winning his state’s governorship. Another target was Sen. John McCain (R-Ariz.), who despite a very conservative voting record to that point exited the 2000 presidential primary against Bush with a strong grudge against the president.

Jeffords became the focus of Reid’s efforts during the tax-and-budget battle in spring 2001, as the size of Bush’s first tax cut kept growing in the Republican-dominated House. Jeffords wanted to rein in the size of the tax cut while also securing funding for special education. Sen. Christopher J. Dodd (D-Conn.), a senior member of Jeffords’s committee, began working with Jeffords, letting him know that if Democrats were in charge, more funds would flow to his favored programs.

Jeffords sided with Democrats on a key amendment that knocked $450 billion off the tax cut, sending more money to education programs, and according to an Associated Press account at the time, signaled that he was ready to make the leap. Reid began actively courting Jeffords in private huddles.

One key sticking point: a chairman’s gavel. Sen. Edward M. Kennedy (D-Mass.), the longtime ranking member of HELP, was his party’s leading spokesman on domestic issues, and no one expected him to step aside as chairman if Jeffords switched caucuses. Nor did Kennedy offer. At the time, Reid served as ranking member of the Environment and Public Works Committee, a key perch but one that, if Democrats held the majority, he would not have much time for because he would be running floor operations for Daschle.

As a final carrot, Reid gave up his top spot on the committee, and Jeffords, once he switched, became chairman.

There was a last-ditch effort to keep Jeffords in the fold by the other four GOP moderates — Chafee and Sens. Olympia J. Snowe (Maine), Susan Collins (Maine) and Arlen Specter (Pa.) — but it failed.

“I feel as if a weight has been lifted from my shoulders,” Jeffords told supporters in Burlington as he announced the switch.