Baton Rouge Congressman Bill Cassidy soundly defeated Democratic U.S. Sen. Mary Landrieu in her bid for a fourth term in the runoff election Saturday, adding to the string of Republican victories across the country this year that gave his party control of the U.S. Senate.

Landrieu’s loss, by about 12 points, ends an era — more than one, actually. It closes out her 18-year run in the Senate, and — at least for the time being — her nearly uninterrupted streak of 34 years in state or federal elective office. And for the first time since the echoes of Reconstruction faded out, no Democrat will represent Louisiana in the U.S. Senate (Republican David Vitter, first elected in 2004, holds the state’s other Senate seat).

Cassidy received 712,330 votes, or 55.9 percent of the ballots cast. Landrieu had 561,099 votes.

Cassidy told his supporters that the vote showed the country’s voters want to go in the conservative direction. “Where we the people have the power and not the federal government,” Cassidy said, calling out Vitter as one of his earliest and most ardent supporters.

Cassidy also thanked volunteers who came in from all over the country. “This was an American victory,” he said.

In her concession speech, Landrieu thanked her supporters for being at her side fighting for the people of Louisiana.

“Where I come from, as you can tell a little bit with this family, there is no quit,” Landrieu said. She then recited Theodore Roosevelt’s famous quote about credit going to the person in the arena.

“The joy has been in the fight,” Landrieu said. “Louisiana will always be worth fighting for.”

The campaign was the most expensive in Louisiana history, with the candidates combining to raise more than $32 million in direct contributions. Millions more were spent by outside political groups who filled the airwaves with strident, mostly negative advertisements.

Landrieu’s own political future is undetermined. She could run for governor in 2015, a race that Vitter and several other well-known candidates have entered. Or she could try to return to the Senate in 2016.

But the prospects for any Democrat running statewide in Louisiana appear dim for the foreseeable future. Landrieu, in her 2008 re-election to a six-year term, was the last Democrat to win a statewide race. The state voted overwhelmingly for Republican presidential candidates in 2008 and 2012.

The beneficiary of the rising Republican tide is Cassidy, 57, who was a full-time physician in the state’s charity hospital system before winning a state Senate seat in 2006. He was elected to Congress in 2008 and is in his third term in the House, where he has registered a solidly conservative voting record but is far from a tea party firebrand.

Cassidy ran a low-key, risk-adverse campaign — strikingly so, for a challenger to a three-term incumbent — on the apparent calculation that in a year that favored Republicans, he need only avoid mistakes to win.

In a speech Friday — the first day he appeared in public in the campaign’s final week — Cassidy turned into a call-and-response chant the overriding theme of his challenge to Landrieu: that she supports Democratic President Barack Obama “97 percent of the time!” based on her Senate voting record and Obama’s agenda. That was a tune played by Republican Senate candidates in many states, as they sought to capitalize on the president’s low approval ratings.

The result was that, with Cassidy’s win, Republicans gained nine seats in the Senate. That outcome fit a long-standing pattern of congressional gains in midterm elections in a president’s second term by the party on the other side of the partisan divide from the president.

Landrieu, 59, is regarded as one of the most conservative — or least liberal — Democrats in the Senate. She campaigned vigorously in an effort to shift the focus away from Obama, emphasizing her record of winning federal benefits for the state, including projects, grants and policies.

Landrieu touted the influence in the Senate earned by her seniority, leading with her ascension early this year to the chair of the Energy and Natural Resources Committee, with is oversight of the oil and gas industry that is central to the Louisiana economy, which Landrieu long has championed. But the Republican wins in other states Nov. 4 meant she would lose her chairmanship of the committee in the 2015-16 Senate — and with it, much of her clout.

Landrieu was forced to play defense early in the campaign, when her effective relocation with her family to Washington after her first election, in 1996, drew national publicity. More damaging were disclosures that she spent money from her official, taxpayer-financed Senate office account to charter planes for campaign trips, in violation of Senate rules. She ultimately paid back to the U.S. Treasury from her campaign funds more than $30,000, after her internal investigation uncovered dozens of wrongful billings dating back to 2002 that she attributed to “bookkeeping errors.”

In the closing weeks of the campaign, Landrieu fired back at Cassidy as “Dr. Double-Dip” — a term Cassidy himself interjected as a rueful comment during their last televised debate. After his election to Congress in 2009, Cassidy worked part-time for the LSU health system until taking a leave of absence this year for the Senate campaign, and time sheets he filled out — and that surfaced in recent weeks — appeared to be incomplete and suggested possible discrepancies with his recorded activities in the House in Washington. Landrieu accused him of potentially committing payroll fraud. Cassidy denied any impropriety.

The handwriting was on the wall for Landrieu after Nov. 4, and not just because Republican victories nationwide robbed her of her seniority-and-influence argument, nor because they demonstrated the potency of the anti-Obama message with a typical low-turnout midterm electorate that includes fewer minorities, young people and other traditional Democratic constituencies than in a presidential-election year (on Saturday, 43 percent of registered voters in Louisiana participated in the election, well short of the 68 percent in the 2012 presidential contest).

Under Louisiana’s unique election system, all Senate candidates, regardless of party, appeared on the same ballot Nov. 4, with the Dec. 6 runoff between the top two finishers required if none of the eight contenders won office in the first round by capturing a majority of that vote. Landrieu topped the field, but with just 42 percent of the vote, and with Cassidy right on her heels. The killer for Landrieu was that a second Republican — retired Air Force Col. Rob Maness, of Madisonville — pulled down 14 percent of the vote running as a tea party candidate to Cassidy’s right, and the combined Republican vote equaled well over half the total.

That left Cassidy a straightforward path to victory in the runoff via unification of the Republican vote behind him. Maness endorsed Cassidy a few days after Nov. 4.

Cassidy had received considerable backing before Nov. 4 in spending by national Republican organizations and outside groups on political advertising, and that continued in full force up to the runoff. Landrieu was strongly supported by the Democratic and progressive counterparts of those groups before Nov. 4 — but whey they saw the results, they all but abandoned her.

In the end, too many planets aligned against Landrieu. A key factor was the dramatic erosion in her support among white voters. In 2008, according to exit polls, one in three white voters supported her, and she ran 12 percentage points ahead of Obama in Louisiana. This year, one in six white voters chose Landrieu. Even with near-unanimous support from black voters, that wasn’t enough.

Elizabeth Crisp of The Advocate Capitol news bureau contributed to this report. Follow Gregory Roberts on Twitter, @GregRobertsDC. Follow Elizabeth Crisp on Twitter, @elizabethcrisp.