Following Tuesday’s final special election since Donald Trump took office, and with the 2018 midterm primaries winding down, the results signal a major concern for Republicans heading into the fall races: turnout.

Data indicate that the unique group of voters who swept Trump into office has not maintained its 2016 levels of enthusiasm, while Democratic fervor continues to rise. Coupled with the president’s job approval rating, which sits at 43.4 percent in the RealClearPolitics polling average, the dynamics resemble those President Obama experienced in 2010 when the GOP’s Tea Party wave flipped control of the House. The Obama coalition of 2008 collapsed, and it appears Trump’s is on the same path.

In all of the 11 special election races — with the exception of Texas’ 27th Congressional District and California’s 34th, due to unique voting systems — Republican turnout was lower compared to the past two election cycles, to varying degrees. In some cases, as in Georgia’s 6th District, GOP turnout was only down by about 4,000 votes, but in Ohio’s 12th District and Alabama’s Senate election, the numbers were far higher.

In OH-12, Republican candidate Troy Balderson -- who holds a narrow lead in the still-incomplete vote count -- received nearly 50,000 fewer votes than did the GOP candidate in 2014, incumbent Pat Tiberi. (The seat opened up when the nine-term congressman resigned to lead the Ohio Business Roundtable.) Meanwhile, Democratic candidate Danny O’Connor received nearly 40,000 more votes than the Democratic candidate in 2014. In Alabama, former Judge Roy Moore received nearly 145,000 fewer votes than Jeff Sessions did in 2014, when the current attorney general was unopposed. A key factor in 2018: The special election to replace Sessions was fraught with allegations of sexual misconduct against Moore after he beat appointed Sen. Luther Strange in the primary. Clearly, no two races are exactly alike, but these data points are only the latest examples of a downward trend in Republican turnout.

Conversely, those three contests all showed huge turnout increases for Democrats. These were also the races that received the most national attention and where outside groups poured in the most money. The Georgia special election to replace Tom Price – who subsequently resigned the Cabinet appointment that created the open seat -- remains the most expensive House race in U.S. history.

Special election races are unique, of course, but they can also be early indicators of how voter sentiments are shifting. Ahead of the first midterm races under Obama, Democrats lost a Senate seat in deep-blue Massachusetts — a perch the late Ted Kennedy had held for decades — to Republican Scott Brown. They also lost a House seat in Hawaii’s 1st District and narrowly held New York’s 20th District by a few hundred votes despite winning it by a double-digit percentage-point margin the year before.

Obama, like Trump, campaigned for many of the Democratic candidates, but he could not incentivize voters to turn out for others within the party as they had done for him. “When [Obama] wasn’t on the ballot, Democrats got crushed. He had a unique appeal that wasn’t easy to duplicate,” said Nathan Gonzales of Inside Elections. Trump, he noted, has a similar problem, and GOP candidates’ attempts to mimic his political style and rhetoric have fallen flat: “No one else is President Trump. To say he is a unique political figure is an understatement.”

Gonzales said a major difference between the two sets of special elections can be seen in the reason voters didn’t turn out for one party. With Obama as president, he said that Democratic voters — particularly minority and younger ones — didn’t cast ballots because they weren’t engaged. Republicans, however, are engaged but seem not to have liked their options.

The head of one conservative group agreed that voter turnout for Republicans will be an issue in November and added that the president remains the best hope to drive up turnout. Trump needs to “snap the base out of their comatose” state in many of these races, said David Bozell, president of ForAmerica. “The issue going forward is he can do that very successfully … but he can’t be everywhere at once. He’s going to these districts saying, ‘Come out for me.’ And people are coming out for him, but that can’t necessarily be replicated all across the country all the time.”

Bozell said it’s Republicans’ failure to act on the president’s agenda, specifically on Obamacare and immigration, that has created a lack of motivation for GOP voters. “It’s just political malpractice on the part of members of Congress not to address these issues,” he said.

Job approval rating is another early indicator of midterm performance for the president’s party and one that Obama in 2010 and Trump in 2018 also share. Eight years ago, Obama’s rating sat at 47.3 percent, somewhat better than Trump’s current rating. But Obama’s number continued to drop heading into the fall elections, where Republicans had a net gain of 63 House seats and six in the Senate (enough to flip the House but not enough to gain the majority of the upper chamber). Four years later, they would win control of the Senate with a nine-seat net pickup -- the largest gain in the chamber by either party since the GOP’s 1994 wave during Bill Clinton’s presidency. Clinton also had a low approval rating -- 41.7 percent -- just before the midterms that year.

Obama, like Trump, remained popular within his own party despite low national approval ratings. Obama averaged an 83 percent approval rating among Democrats during his eight years in office, according to Gallup. A Gallup poll in June showed Trump at 88 percent approval among Republicans. At the same time, however, the generic ballot was, and is, creating headwinds for both men. Republicans were favored ahead of the 2014 midterms by 2.5 percentage points and ahead of the 2010 races by 9.4 points, according to the RCP average. The current generic ballot shows Democrats are favored by five.

The direction these numbers signal is not a surprise to most election watchers. “Voters are showing up as you would expect them in a lot of areas in these races,” said Brandon Finnigan, founder of Decision Desk HQ. He pointed out that motivation is always higher for the party out of power and voters tend to treat midterm races as a referendum on the party in the White House.

If Republicans do have some hope on the numbers front it’s that, overall, their voters historically have better turnout in midterm races than Democrats. FiveThirtyEight calculated that the GOP holds a three-point advantage in midterm years since 1978. This is something Democrats have tried to focus on since the 2016 election and a Pew research survey from July shows that the number of Democratic votes cast this year in House primary races is 84 percent higher than at this point in 2014. The survey also found Republican votes in House primary races are up, but only by 24 percent.

Even this has to be taken with a grain of salt, however. A Gallup survey from June noted that those who say they are “absolutely certain” they will vote is only 56 percent -- similar to the 58 percent margin recorded just before the 2014 midterms, which was the lowest turnout since 1942.