If President Obama’s 2012 campaign was known for anything, it was its voter mobilization operation, said to be the most sophisticated ever assembled in a presidential campaign. Which makes David Plouffe’s comments over the weekend all the more telling for Democrats as they look nervously toward the November midterm elections.

When Republican David Jolly defeated Democrat Alex Sink in Florida’s 13th Congressional District a week ago, Democratic leaders explained away the outcome by arguing that the district tilted heavily in favor of the GOP in midterm and especially special elections. Rep. Steve Israel (N.Y.), who chairs the Democratic Congressional Campaign Committee, said that Republicans had a 13-point turnout advantage and that Democrats had made up almost all of that ground, only to fall a few points short.

Plouffe, who was Obama’s campaign manager in 2008 and oversaw the reelection bid as White House senior adviser, put the onus back on the Democrats. Democrats didn’t lose the special election because the Republicans had an insurmountable advantage in the district. They lost because they couldn’t get enough voters — the ones who backed Obama in 2012 — to the polls. Plouffe called the loss a “screaming siren” for the fall. As he put it, “We have a turnout issue.”

The independent Cook Political Report, which gives every congressional district a partisan index, rates the Florida district only as narrowly Republican in its leaning, an “R-plus-1” rating. Of the 435 districts, the Cook team rates more than 200 as more Republican than Florida’s 13th.

Districts like Florida’s 13th may look more Republican in off-year elections than in presidential years, but as Plouffe pointed out, that’s because Democrats have a turnout problem in those midterm elections. The Democrats’ coalition includes groups of voters who are simply less likely to show up for midterm elections. Younger voters turn out at lower rates in midterm elections than older voters. Single women are less likely to vote than married women.

View Graphic Inside the 36 Senate contests of 2014

At the beginning of each midterm election cycle, Democrats vow to do a better job of getting their voters to the polls. But when history (a president’s party generally loses seats in midterm elections) and the political winds are blowing in the wrong direction, they’ve fallen short.

That was the case in 2010, when Republicans made historic gains in the House just two years after Obama and the Democrats celebrated his 2008 victory as a sign that the pendulum was swinging permanently in their direction.

After the government shutdown in October, Democrats told themselves that the Republicans were in such poor shape that the House could actually change hands with the 2014 contest. No one is suggesting that today, which may be one reason such longtime Democratic stalwarts as Reps. John D. Dingell (Mich.) and Henry A. Waxman (Calif.) have decided to retire. Republicans are favored to hold their House majority, and Democrats are looking mainly at holding down their losses.

The Senate is another story. Former Obama White House press secretary Robert Gibbs said Sunday on NBC’s “Meet the Press” that Democrats should worry that the electorate in November will look more like it did in 2010 than in 2012. If that’s the case, he said, the GOP could win control of the Senate. “If we lose the Senate, turn out the lights,” he said, “because the party’s over.”

Gibbs had uttered something similar about the possibility of big losses in 2010 and was slapped down by senior Democrats, including then-House Speaker Nancy Pelosi (Calif.).

Gibbs quipped Sunday that he still has “tire tracks” on his body from that experience. But his point is a serious one. Democrats must get their voters to the polls in November or risk losing control of the Senate, which would make life even more difficult for Obama during his final two years in office than it has been with Republicans in control of the House.

Democrats have a turnout problem because they have a motivation problem. In 2010, their biggest problem was that they ran into an energized GOP electorate. The rise of the tea party movement and hostility toward Obama’s health-care law brought Republicans to the polls while Democrats stayed home. The president’s vaunted political operation seemed powerless in the face of that aroused opposition.

What Democrats learned in 2010 was that Obama’s personal appeal was not transferrable to Democrats running for the House or the Senate. Plouffe and others said at the time that it was essential for Democratic candidates to develop their own relationships with voters and not rely solely on the Obama organization to turn out voters. Some did; many did not.

A companion problem is confronting Democrats this year: dissatisfaction in their ranks. Obama has disappointed many of his followers, and his overall approval ratings are low enough to give the party real concern. The lack of enthusiasm for the president could easily lead to demoralization and too many stay-at-home voters in November.

Republicans again are motivated by their dislike of Obama’s health-care law and by the prospect of controlling both chambers of Congress. Republican National Committee Chairman Reince Priebus said Tuesday morning that the Affordable Care Act is “total poison” for Democratic candidates this year.

More than enough Senate seats are in play to put the Democrats in obvious danger of losing their majority. Many of the most vulnerable seats are in states Obama lost in 2012, making his ability to help those candidates — beyond fundraising — extremely limited.

The president can attempt to frame this coming election as a choice, not a referendum, as he was able to do in 2012 against Mitt Romney. But what works for his overall coalition could be less effective with electorates in red states. He can press ahead with initiatives designed to motivate core parts of his coalition — on climate change or social issues, for example — but will those work as effectively in Senate races in red states as in a national election?

The Democratic Senatorial Campaign Committee foresaw the midterm election turnout problem some time ago and began to invest in a substantially bigger effort to get voters to the polls in states with contested Senate races. But in the past few months, the Senate map has expanded, meaning the party will need even more money than before to implement the program.

Democratic campaigns will have many of the same tools the Obama campaign used in 2012, but tools alone are not sufficient without a motivated electorate. That’s why Plouffe and others have begun to sound the alarms.