Now that Louisiana’s voters have added their crushing coda to this year’s Republican sweep, many of the ways in which next year’s Senate will be different have locked in place.

The most obvious change has been known since election night: The GOP will be in charge for the first time in eight years. But now we know Republicans will occupy 54 seats starting in January, strength in numbers they’ve exceeded in only six years of the previous three decades.

Beyond that, the defeat of Democrat Mary L. Landrieu — she took just 44 percent and lost her bid for a fourth term representing Louisiana by 151,000 votes in a runoff against GOP Rep. Bill Cassidy — will further shift senatorial demographics and political dynamics on several fronts.

For all the hay the GOP made during the campaign about how Landrieu backed President Barack Obama 96 percent of the time in her current term, which proved a simplistic but effective kiss of death in her deeply red state, it’s undeniable she was among the most senior centrists remaining in her caucus. Her ouster shrinks to single digits the number of Democrats who might ever be persuaded to embrace a deal driven by the Republicans, given that 2014 already had been marked as the exit date for four party moderates. (Mark Begich of Alaska, Mark Pryor of Arkansas and Kay Hagan of North Carolina were defeated, while Max Baucus of Montana left early to be ambassador to China.)

The departures of those five will make it all the more difficult for either Obama or incoming Majority Leader Mitch McConnell to practice bipartisan triangulation if they choose to replace gridlock with accomplishment as their byword for the 114th Congress. As a result, at least in the near term, the decline of the moderate conservatives will do more to change the way the Senate works than the historic milestone that’s been getting so much attention in recent days.

That is, Landrieu’s loss, on top of Rep. John Barrow’s defeat in Georgia a month ago, heralds rock bottom for the people who dominated political power in the Deep South for much of the past century. Next year, that region won’t send a single white Democrat to Congress. (In addition, Louisiana won’t have any Democratic statewide-elected officials for the first time since 1876.)

Things don’t get much better for the Democrats, even if you take the word “white” out of the formulation and expand the definition of the South beyond the six states in the heart of Dixie to include all of what political scientists call the Solid South, the 14 states that were almost uniformly Democratic in their voting between the end of Reconstruction and the enactment of the Civil Rights Act half a century ago. The midterm elections means 24 of this region’s senators (86 percent) will be Republican next year. (The only exceptions are Florida’s Bill Nelson, West Virginia’s Joe Manchin III and Virginia’s Tim Kaine and Mark Warner.)

The GOP’s dominance of the region in the House is almost as emphatic. Because of a net pickup of two districts in the South, the party will hold 114 of the region’s seats (75 percent) after swearing-in day — and that bloc will account for 46 percent of the House Republican Conference. (Fourteen of the 38 Democratic members from the more expansive South will be white.)

Landrieu’s departure also finalizes this year’s shift in the Senate’s gender balance. Because she and Hagan are leaving, while Republicans Joni Ernst of Iowa and Shelley Moore Capito of West Virginia are arriving, the share of Democrats among the female senators is slipping to 70 percent from 80 percent, while the GOP share is jumping to 30 percent from 20 percent.

The year’s last election outcome also finalizes the depth of the seniority drain on the Democratic side. Landrieu’s 18 years in office get added not only to the 30 years of combined service from the party’s four other defeated incumbents, but also to the 178 years of cumulative service accrued by the half dozen Democratic senators voluntarily departing between the beginning and the end of the 113th Congress.

Landrieu took the gavel of the Energy and Natural Resources Committee this year, a promotion from her full committee chairmanship of the previous five years at Small Business. She’s also spent her past four years on Appropriations as chairwoman of the Homeland Security Subcommittee. Because she’s leaving, the Democrats now probably have at least three seats to fill on each panel and on her fourth committee, Homeland Security and Governmental Affairs. (Washington’s Maria Cantwell will step up to the party’s ranking seat on Energy, a bittersweet development for many in the caucus who were uncomfortable with Landrieu’s full-throated and parochially mandatory allegiance to the oil and gas industry.)

Cassidy won’t know his committee assignments for a while yet, but his victory has already affected the nature of the Senate GOP. A gastroenterologist, he will keep at three the number of physicians in his caucus, joining ophthalmologist Rand Paul of Kentucky and orthopedic surgeon John Barrasso of Wyoming. (Obstetrician Tom Coburn of Oklahoma is retiring; there are no medical doctors among the Senate’s Democrats.)

Having spent three terms on the other side of the Capitol, Cassidy will join six current House colleagues in crossing the Rotunda in four weeks — bringing to 53 the number of former House members in the new Senate. That’s close to a record in a chamber where, during the past two centuries, only one-third of members also have served in the House.

And with Casssidy’s arrival, the Senate GOP freshman class will number a dozen — meaning 34 of next year’s Republicans, or 63 percent, will begin 2015 never having experienced senatorial life in the majority. That collective form of inexperience could work in McConnell’s favor, or to his detriment, as he sets about trying to build legislative bridges that unite the establishment and confrontational halves of his caucus.

Related:

Louisiana Runoff Results: Landrieu Loses

McConnell Plots Functional, Bipartisan Senate

Roll Call Casualty List for the 113th Congress

The 114th: CQ Roll Call’s Guide to the New Congress

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