In no case did these majorities lead to long-term control of the federal government. Messrs. Clinton, Obama and Trump saw their parties lose one or both houses of Congress two years after taking office. In 2001, Mr. Bush’s Republicans lost their Senate majority in five months when Vermont Sen. Jim Jeffords switched parties. The GOP regained its Senate majority in 2002 and 2004, in both cases picking up a seat in Georgia.

For nearly two decades, Georgia seemed an impenetrable Republican stronghold. The party’s candidates won every presidential, gubernatorial and senatorial race there from 2002-18. But this year, affluent voters in metro Atlanta, like their counterparts in Houston, Dallas and Phoenix, started inching away from the Republicans, as affluent voters in the Northeast, Midwest and West Coast did starting in the 1990s. An increasing black population—many moving from cities like Chicago and Detroit—has contributed to the rising Democratic tide across the South. Georgia, Texas and Arizona are now presidential target states.

Voters in Georgia’s cities and suburbs recoiled against Mr. Trump, for reasons of both style and substance. He lost Georgia to Mr. Biden in November by a margin of 49.5% to 49.3%, then he lost his party’s two Senate seats in the runoff.

Tuesday’s result wasn’t inevitable. In November Sen. David Perdue edged Democrat Jon Ossoff 49.7% to 47.9%. In the special all-party election for the vacancy created by the resignation of Sen. Johnny Isakson, Republican candidates led Democrats by an almost identical margin: 49.4% to 48.4%. But under Georgia law, Mr. Perdue’s failure to win an absolute majority put him in a runoff against Mr. Ossoff. Sen. Kelly Loeffler, appointed to replace Mr. Isakson, squared off in a head-to-head battle with the lead Democrat, the Rev. Raphael Warnock.

Enter Mr. Trump. His unhinged challenge to the results of Georgia’s presidential count, his vitriolic attacks on Gov. Brian Kemp and Secretary of State Brad Raffensperger —both Republicans—and his charges that the state’s voting machines were rigged split the state’s Republicans. His sudden demand that the Covid relief bill include checks for $2,000 rather than the $600 agreed to by his Treasury secretary put the two Republican senators on the defensive.

Many Georgians who were open to voting Republican were appalled by what they heard on a recording of Mr. Trump’s weekend phone call with state officials. The president demanded that Mr. Raffensperger “find” the 11,780 votes that would give him Georgia’s 16 electoral votes.

Turnout in Tuesday’s runoff was 89% what it was in November—and even higher among blacks—but was held down in the state’s pro-Trump northwest counties by the president’s insistence, repeated at an election-eve rally there, that Georgia’s voting apparatus had been hopelessly corrupted. As of press time, Mr. Warnock has defeated Sen. Loeffler 50.6% to 49.4% and Mr. Ossoff has beat Mr. Perdue 50.2% to 49.8%.

Mr. Trump isn’t the first candidate to charge Georgia Republicans with cheating him out of an election. Democrat Stacey Abrams made the same charge after she lost 50.2% to 48.8% to Mr. Kemp in 2018. Democrats may have profited during the current cycle from Ms. Abrams’s absurd charges that black voters in Georgia were being disenfranchised and “suppressed.”

Mr. Trump’s claims, on the other hand, appear to have cost Republicans a majority in the Senate and created fissures that could prevent them from regaining congressional majorities in 2022. Whether they can bounce back may depend on the behavior of the man whose unforced errors are responsible for their current predicament.

Mr. Barone is senior political analyst for the Washington Examiner and founding co-author of “The Almanac of American Politics.”

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