With characteristic energy, Sen. Mary Landrieu was up bright and early the day after the primary election, slamming her opponent Bill Cassidy.

The lack of even a day or two of rest shows that Cassidy is the real frontrunner in the Dec. 6 runoff for a Senate seat.

Landrieu, a three-term incumbent, finished first in the primary but at 42 percent of the vote was well short of re-election. And Cassidy with third-place finisher Rob Maness, also a Republican, posted a majority of the vote.

That adds up to a formidable challenge to Landrieu’s long run as a senator.

Both campaigns, though, are right to stress that the December race is not a case of a re-run of the primary, but is a separate election, and the dynamics of the race may be different.

With the new Senate majority to be Republican, whatever the outcome in Louisiana, Landrieu’s clout in the Senate is diminished. Still, she has delivered major benefits to the state, in the Senate minority or majority, whether a Republican or Democrat is in the White House.

That experience and record is her asset in the runoff, set against Cassidy’s ability to rouse Republican-leaning voters on national themes. The voters of Louisiana were casting votes Nov. 4 as much or more against President Barack Obama than Landrieu, and Cassidy seeks to rally the troops for an extra kick in the presidential pants.

Is “Obamacare” the most vital issue facing Louisiana? For motivated Republican voters, probably so, but those were pretty much lost to Landrieu from the beginning.

Now, as Cassidy’s backers are just as insistent in saying as Landrieu’s, it’s a new ballgame. The Republican seeks to ensure turnout of the challenger’s supporters and those who were of similar, if not the same, ideological leanings as those backing Maness.

Again, though, Cassidy runs in the posture of an incumbent, agreeing to only one face-to-face debate Dec. 1. That is minimal exposure to the chance of a mistake in a race that Cassidy believes is his to lose. His enemy is complacency in his own camp.

Geography may work in his favor: As a longtime physician in Baton Rouge’s Earl K. Long charity hospital, and then taking a turn in the state Senate and five years in a Baton Rouge-based district in the U.S. House, Cassidy has a base in the capital. And that may be the key metro area in the race.

There are votes for Landrieu in East Baton Rouge, but much of the 6th District is Republican, and GOP nominee Garret Graves will be pushing the turnout of Republican voters in the runoff against former Gov. Edwin W. Edwards.

Elsewhere, will there be the turnouts to help Landrieu make up ground? There is a mayoral runoff in Shreveport but a relatively thin ballot — a few taxes, a few runoffs for local races — in most other parishes. In Orleans Parish, her hometown, there is little else on the ballot.

While the 5th District has a runoff for Congress, both parties are likely going to be pushing turnout, and it is majority white and tends to elect Republicans. Still, Monroe Mayor Jamie Mayo’s candidacy may give Landrieu a boost by turning out a few more of her voters also backing the African-American mayor.

Landrieu also faces a challenge in the calendar: She is busy with the Senate session, although it is generating some good press for her fight with Obama on the Keystone pipeline project. Many of the white voters she so clearly failed to convince in the primary are in rural areas and small cities. In some places, she has a case that she has delivered for them, but making that case requires retail campaigning that will be time-consuming in such a short-fuse race.

Lanny Keller is an editorial writer for The Advocate. His email address is lkeller@theadvocate.com.