Three-term Democratic U.S. Sen. Mary Landrieu was forced Tuesday into a Dec. 6 runoff showdown with challenger U.S. Rep. Bill Cassidy, a Republican who pressed Landrieu closely in total votes on a night when his party’s candidates across the country soundly beat enough Democrats to take control of the Senate.

With all 4,018 precincts reporting, Landrieu took 618,840 votes, or 42 percent, to Cassidy’s 602,439, or 40.9 percent. Tea-party Republican Rob Maness took 202,413 votes, or 13.7 percent, to keep either Landrieu or Cassidy from the majority needed to win outright Tuesday. Five other candidates split the remainder of the vote.

The Republicans success Tuesday in winning enough Senate seats in other states to seize control of the Senate will shape the contours of the Landrieu-Cassidy runoff.

Outside political groups already have spent millions for TV and radio ads to influence the election. With the Dec. 6 vote the only one left in the nation for a Senate seat, the race is likely to attract the focused attention of both Democrats and Republicans, although possibly less than if the outcome held the key to control of the Senate. The spending is sure to increase.

In an election night speech to supporters in New Orleans, Landrieu struck a combative tone for her runoff campaign.

“We have the race that we want, and Bill Cassidy, you can’t run, you can’t hide anymore,” she said. “This race is starting tonight.”

Landrieu recounted several times where she has worked to benefit Louisiana residents, including Hurricane Isaac relief, flood protection in Louisiana and compensation payments for crawfish processes for illegal dumping of Chinese imports. Each time she asked “where was Bill?” and the crowd picked that up as a chant.

She challenged Cassidy to six debates. “You’re going to have to say, Mr. Cassidy, more than President Obama’s name in these six debates,” she said.

In her three previous elections, Landrieu twice was forced into a runoff, and prevailed. In 1996, she trailed Republican Woody Jenkins in the open primary but edged him by 5,788 votes in the run off, a margin she has memorized and frequently repeats in campaign appearances. In 2002, Landrieu led Republican Suzanne Haik Terrell in the primary and went on to defeat her in the runoff.

Opinion polls have consistently shown Cassidy defeating Landrieu in a head to head matchup.

Cassidy entered the Baton Rouge hotel room filled with his supporters shortly before 10:30 p.m. to chants of “Bill! Bill! Bill!”

In a theme familiar from the campaign trail, he gave a speech focusing heavily on President Barack Obama, seeking to tie Landrieu’s policies to the president whose popularity has dwindled in Louisiana.

“The people of Louisiana have sent a signal tonight that you want a senator who represents Louisiana, not Barack Obama,” Cassidy said.

He said he received a call from Maness and expects Maness’ supporters to vote for him in the runoff.

“We have 32 more days, this is not over yet,” he said.

Opinion polls before the Tuesday vote showed Landrieu leading the eight-candidate field, with Cassidy close behind. A political newcomer, Maness, an retired Air Force colonel from Madisonville, was the only other candidate to mount a statewide campaign.

Landrieu talked up some of her differences with Obama, while also reaffirming her support for his signature legislation, the Affordable Care Act, known as Obamacare. But the main emphasis of her campaign was on her ability to deliver for the state, in large part via her chairmanship of the Senate Energy and Natural Resources Committee, with its oversight of the oil and gas industry central to Louisiana’s economy — a post she acquired earlier this year by virtue of her seniority among the Senate’s ruling Democrats.

The new Republican majority means Landrieu will be displaced as chair of the energy committee and will be less able to play her “clout” card.

Maness decried both Cassidy and Landrieu as creatures of Washington, and claimed the mantle of the only true conservative in the race. Among House Republicans, Cassidy rates as more conservative than the average, while Landrieu generally scores among the least liberal Senate Democrats.

Landrieu has raised the most money from contributions directly to her campaign: $16.8 million through Sept. 30, based on the latest federal financial reports. Cassidy is second, with $11 million, and Maness is third, with $2.6 million. No other candidate reporting raising as much as $10,000.

Landrieu, 58, has campaigned vigorously around the state, rounding up endorsements and touting her achievements in winning federal money for myriad projects in local communities. She was put on the defensive in mid-campaign by reports that she had misspent taxpayer money on campaign charter flights -- she ultimately paid back more than $30,000 — and by complaints that she doesn’t really live in her parents’ home in New Orleans that she claims as her voting address, but in the house she and her husband built in Washington near the U.S. Capitol.

While waiting for Cassidy to appear Tuesday night at his election party, former Republican U.S. Rep. Joseph Cao, of New Orleans, said “We don’t want a Washingtonian to lead Louisiana. We want a Louisianan to lead Louisiana.”

Landrieu grew up in politics: Her father, Moon, was mayor of New Orleans while she was in high school and college (her brother, Mitch, is mayor now). She was elected to the state House a couple of years after graduation from LSU. Save for one brief interlude, she’s held elective office ever since, including eight years as state treasurer in the 1980s.

Cassidy, 57, has run a lower-key campaign. The candidate of the Republican establishment, he took flak from both Landrieu and Maness for declining to appear in all but two televised campaign debates. He built a career as a physician in Baton Rouge, working in the state charity hospital system and on the LSU medical faculty, before winning election to the state Senate in 2006. He moved up to the U.S. House in 2008 and is in his third two-year term.

While lacking the financial and organizational support of his better-funded rivals, Maness, 52, conducted an active campaign, driving more than 80,000 miles in his Ford F-150 pickup truck and visiting all 64 parishes. He moved to Louisiana permanently in 2011 after a career in the Air Force, retiring as a colonel, and worked briefly as a safety manger for Entergy before quitting to make his first run for elective office. He has been endorsed by some tea-party groups and by Sarah Palin, the movement icon who was the Republican vice-presidential candidate in 2008.

Follow Gregory Roberts on Twitter, @GregRobertsDC. For more coverage of government and politics, follow The Advocate Politics Blog at /politicsblog.