For Democrat Mary Landrieu, the key to her re-election to the U.S. Senate on Dec. 6 is “30/30.”

That’s the formula she needs to meet to overcome the odds and defeat her runoff opponent, Republican Bill Cassidy, and win a fourth six-year term, her campaign staff says.

The formula represents a black voter turnout that equals 30 percent of the Dec. 6 electorate, plus a 30 percent share of the white vote that day: 30/30. The numbers can vary a bit so long as the changes offset one another and the total adds up to 60 or more.

In the open primary Nov. 4, the Democratic turnout effort hit the first target: Black voters made up 30 percent of the electorate, based on exit polls, which is just shy of their share of all registered voters in Louisiana. And they gave Landrieu 94 percent of their votes, according to those polls, which provide a rough measure of voter behavior.

But Landrieu fell dismally short of the second goal: Only 18 percent of white voters supported her, the exit polls showed.

That combination actually was enough for her to lead the open primary field with 42 percent of the vote. But Cassidy, a third-term congressman from Baton Rouge, finished close behind her at 41 percent, and a second Republican, tea-party candidate Rob Maness, pulled 14 percent of the vote. Opinion polls predict that Cassidy will attract enough Maness voters to win the runoff.

The Landrieu campaign deserves credit for turning out black voters in a midterm election, with no presidential race on the ballot to spur interest, said Joshua Stockley, who teaches political science at the University of Louisiana at Monroe.

The black share of the vote was similar to what it was in 2008 and 2012, which were not only presidential years but featured Democrat Barack Obama winning election and then re-election as the nation’s black president.

Overall voter turnout drops off in non-presidential years, and Nov. 4 was no exception, as voter participation declined from 68 percent in 2012 to 50 percent. This year’s rate was higher than the 44 percent in the last congressional midterm election, in 2010, which included the contest for Louisiana’s other U.S. Senate seat. More significantly for Landrieu, the black share of the midterm vote rose from 27 percent in 2010, paralleling or exceeding the black growth in the share of all registered voters since then.

“That’s the part of Sen. Landrieu’s campaign strategy and organization that clearly worked well,” Stockley said.

But duplicating that success on Dec. 6 is no sure thing. To begin with, national Republican organizations are pouring resources into supporting Cassidy — both money for campaign commercials and staffers to reinforce their own turnout effort. The national Democratic commitment to Landrieu so far is considerably weaker and more equivocal.

Landrieu loyalists point to 2002, the last time she won a Senate runoff (she was re-elected in 2008 by taking a majority of the primary vote). Landrieu, seeking her second six-year term, led the 2002 primary field by a wide margin, although her 46 percent of the vote was less than the total captured by the four Republican candidates. She beat Republican Suzanne Haik Terrell in the runoff, with the black share of the total vote increasing slightly from the primary.

Democrats also hope their turnout effort will get a lift from the scheduling of the runoff on a Saturday, as opposed to the open primary, held on a Tuesday.

But for Landrieu, the far more daunting challenge is increasing her share of the white vote.

One tactic would be to reach out to white Democratic voters who skipped the Nov. 4 primary and motivate them to show up at the polls Dec. 6.

“She can’t win on blacks alone,” Stockley said. “She does need some white turnout. But the campaign is going to have to be very selective as to where that turnout occurs. The campaign is going to have be very precise.

“You don’t care about independents,” he said. “You’ve got to get your base out.”

The problem for Landrieu is that — despite a continued Democratic advantage in voter registration statewide — that base has been shrinking dramatically among white voters, in Louisiana and across the South.

“What’s going on there is the culmination of a process that’s been happening for quite a while,” said Pearson Cross, a political science professor at the University of Louisiana at Lafayette. “Whites in the South have been leaving the Democratic Party, or at least not voting Democratic, in increasing numbers — really, ever since Barry Goldwater.”

Goldwater, a Republican senator from Arizona, ran for president against incumbent Democrat Lyndon Johnson in November 1964 — a few months after Johnson had signed the landmark Civil Rights Act into law.

Johnson remarked to an aide after the signing, “I think we just delivered the South to the Republican Party for a long time to come,” and in November, Goldwater won the five Deep South states of Louisiana, Mississippi, Alabama, Georgia and South Carolina while suffering a landslide nationwide defeat.

With the exception of Dwight Eisenhower in Louisiana in 1956, no Republican presidential candidate had carried a Deep South state since the end of Reconstruction in the 1870s. The “solid South” was solidly Democratic, controlled by white Democrats who systematically disenfranchised black voters and battled fiercely against efforts to end the racial segregation imposed by state laws.

But Goldwater’s success marked a turning of the tide, as the national Democratic Party embraced civil rights and other liberal causes unpopular with Southern white people. With the exceptions of former Georgia Gov. Jimmy Carter in 1976 (and in his home state in 1980) and of Arkansas Gov. Bill Clinton in 1992 and 1996, no Democratic presidential candidate has carried even one Deep South state since 1960.

For the most part, the presidential pattern has gradually extended to other federal offices and to state elections, and throughout the 11 states of the old Confederacy.

“We’re seeing it working its way down the ticket until all the state legislatures in the Deep South and in most of the South are Republican-dominated, and most of the state offices are Republican,” Cross said.

“White candidates running as Democrats for the Senate or Congress can count on very few white voters in the South,” he said. “It makes the possibility of them winning election very slight.”

Should Landrieu lose Dec. 6, the new Congress convening in January will include no white Democrats from the Deep South. The region’s only Democrats in Washington will be black House members such as Cedric Richmond, of New Orleans, who represent black-majority districts drawn to comply with federal voting-rights laws.

The trend seems to have intensified since 2008, Cross said. In that year, both houses of the Louisiana Legislature were controlled by Democrats (Republicans now hold sway in Baton Rouge), and the lieutenant governor was a Democrat: Mitch Landrieu, the senator’s brother.

Obama that year took less than 40 percent of the presidential vote in Louisiana, but Mary Landrieu was re-elected with the support of one-third of white voters. She was the last Democrat to win statewide in Louisiana.

“Since 2008 and the election of Barack Obama, the number of proud white Democrats who vote Democratic seems to have shrunk and shrunk,” Cross said.

Obama barely improved on his Louisiana numbers in 2012, and he remains deeply unpopular in the state — and is widely unpopular nationwide. Cassidy has pounded relentlessly on Landrieu’s consistent support for Obama’s agenda, a tactic mirrored by Republican candidates in other states in their successful effort to gain enough Senate seats to take over the majority.

The Landrieu campaign hopes that with the issue of Senate control resolved — even if it is in the Republicans’ favor — voters will be less motivated by anti-Obama hostility. Since the primary, Landrieu has attacked Cassidy aggressively in an effort to focus the campaign on a direct comparison between the two of them, which she feels will benefit her because of her years of delivering federal largesse to Louisiana.

Michael McDonald, a political scientist at the University of Florida, thinks Clinton succeeded as the last Democrat to win in the Deep South because of his populist message. But with wages stagnating even as the economy recovers, that’s a tough sell for Democrats today, he said.

As a Democrat running for election in the Deep South in 2014, Landrieu clearly is swimming against a strong historical tide.

“The Democrats failed to keep the South relevant,” Stockley said. “They thought, as one author put it, that they could whistle past Dixie.”

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