Finishing his speech by saying “Boom!” U.S Rep. Bill Cassidy defeated three-term incumbent Mary Landrieu to become Louisiana’s second Republican in the U.S. Senate.

With 3,897 of the state’s 3,931 precincts reporting, Cassidy, a Republican from Baton Rouge, had 702,087 voters or 56.78 percent of those cast. Three-term incumbent U.S. Sen. Mary Landrieu, a Democrat from New Orleans, had 534,331 votes tallied.

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.

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

Republican U.S. Sen. David Vitter, now the state’s senior sentator, received special mention from Cassidy for his help.

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. “We may not have won tonight, but we have certainly won some extraordinary victories.”

Landrieu said the fight was not over on healthcare. “We have to have a healthy workforce,” she said, quoting President Theodore Roosevelt and noting she has been in the arena for 34 years.

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

The Associated Press called the race for Cassidy at 8:25 p.m.

Gov. Bobby Jindal then tweeted “It’s about time. We’ve finally retired Mary Landrieu.”

Landrieu is the last Democratic senator in the Deep South and is the last Democrat to have been elected statewide in Louisiana.

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, 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 President Barack 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 — and which Landrieu has long 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.” But Republicans stuck her with an “Air Mary” label right up to the end.

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, and for the Landrieu campaign, the would-be scandal was too little, too late.

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 — he credited Vitter for providing important advice and guidance to his candidacy.

In that appearance, Cassidy turned into a call-and-response chant the overriding theme of his challenge to Landrieu: that she supports Democratic President 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 and widespread dissatisfaction with his signature legislative achievement, the 2010 Affordable Care Act — conveniently, for Republican purposes, known as Obamacare.

The result was that, with Cassidy’s win, Republicans gained nine seats in the Senate, turning a 55-45 deficit in the current Congress to a 54-46 majority in the one taking office in January. That outcome fit a longstanding 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, 58, 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 — and which Landrieu has long 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.” But Republicans stuck her with an “Air Mary” label right up to the end.

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, and for the Landrieu campaign, the would-be scandal was too little, too late.

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.

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 41 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 and joined him at campaign appearances, followed by such national tea-party icons as Sarah Palin, U.S. Sen. Rand Paul, of Kentucky, and Phil Robertson, of “Duck Dynasty” reality television fame. More mainstream Republicans came to the state to campaign for Cassidy, too, such as Texas Gov. Rick Perry and U.S. Sen. Marco Rubio, of Florida.

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.

Check back later with The Advocate for more details.

Follow Gregory Roberts on Twitter @GregRobertsDC. Follow Elizabeth Crisp on Twitter @elizabethcrisp. For more coverage of the Louisiana Legislature, follow our Politics blog at http://blogs.theadvocate.com/politicsblog .