What's going on?
Analysts say it's too soon to extrapolate too much from the slide. For one thing, depressed turnout in the primary season does not always mean lower interest in the fall - and it's quite possible that participation will pick up for the Feb. 28 primaries in Arizona and Michigan, with the contest suddenly more competitive after former Pennsylvania Sen. Rick Santorum swept three states from Mitt Romney on Feb. 7.
Still, some Republicans are worrying about a possible enthusiasm gap. Their concern goes beyond the White House race; a weak ticket could hurt the party's chances to keep control of the House and pick up ground in the Senate.
"It looks to me there are some warning signs," said Michael McDonald, a George Mason University public-affairs professor and expert on voter turnout.
While the Republican campaign has not lacked for drama, with plot twists and lead changes galore, turnout has declined consistently since the first contests in Iowa and New Hampshire, with the number of Republican and GOP-leaning independent voters who cast ballots falling everywhere but in South Carolina.
The uptick in enthusiasm there came after two closely watched debates and 11 days of furious campaigning, with former House Speaker Newt Gingrich trouncing Romney on Jan. 21. But the voter surge did not continue.
Turnout has fallen in every contest since then compared with 2008 - off 14 percent in Florida, 26 percent in Nevada, 6 percent in Colorado, 23 percent in Minnesota, and 58 percent in Missouri. Obama's campaign has been eager to highlight the falloff.
"Republicans' consistently low turnout shows they are voting with their feet and staying home," Joel Benenson, the Obama campaign's pollster, wrote in a memo to reporters. He argued that the participation rate was a sign of Republicans' "discontent" with their options.
Part of the reason may be the relentlessly negative tone of much of the nominating contest. TV attack ads, after all, are designed to suppress turnout, though they did not seem to hurt so much in South Carolina. (Maybe that bumptious state, where the first shots in the Civil War were fired, has an unusual tolerance for negativity?)
Analysts also say that a lack of excitement for Romney, the former Massachusetts governor, may be contributing to the trend. An analysis by McDonald showed that in two states - Florida and Colorado - turnout rose compared with 2008 in the counties where Santorum and Gingrich did well, even though it declined overall.
"Romney's winning on the basis of the fact that he really has no other candidate contending with him for moderate and independent voters," McDonald said. "He's not really exciting the base, but benefiting from the fact that conservatives are split."
There's "plenty of time for Romney to retool," he said, and whoever wins the GOP nomination still stands to benefit from the natural rallying effect when the fighting is over and "the antagonism is focused on Barack Obama."
Romney's own pollsters say they expect turnout to increase in the most competitive primaries to come, and they point to surveys that show GOP voters still strongly desire to defeat Obama.
It was probably inevitable that intensity among GOP voters would drop from a high point in 2010, said Dante Scala, a political science professor at the University of New Hampshire.
"It's hard to keep up that passion from election to election," he said. At the same time, Scala said, residual disapproval of Obama may not be enough, and Romney will need to fill out a positive vision of his presidency if he wants to consolidate the GOP base and win in the fall.
"You need something more than negativity for motivation in a presidential year," Scala said.
Contact politics writer Thomas Fitzgerald at 215-854-2718, firstname.lastname@example.org, or @tomfitzgerald on Twitter. Read his blog, "The Big Tent," at www.philly.com/BigTent.