In special elections in Kansas and Montana, Democratic candidates spent months waiting for reinforcements in races that national leaders worried were unwinnable. In Georgia, though, the cavalry has arrived. In full force.

An Atlanta Journal-Constitution analysis shows that Democrats have narrowly outspent Republicans in the runoff phase of Georgia's 6th District contest, the nationally-watched race that could prove an early test of the GOP agenda.

The contest, by far the most expensive U.S. House race in the nation’s history, has now cost more than $36 million overall. That includes about $21 million spent or reserved for advertising since April 18, when Republican Karen Handel and Democrat Jon Ossoff notched spots in the June 20 runoff.

National Democratic groups and Ossoff’s campaign have combined for about $11.3 million of that spending – the vast majority on a flood of broadcast TV ads inundating the suburban Atlanta district.

Ossoff’s campaign has now spent or reserved $7.3 million in broadcast, cable and radio ads through the vote. The Democratic Congressional Campaign Committee pumped in another $3.6 million, and the left-leaning House Majority PAC has spent about $250,000.

Republicans have poured in nearly $10 million into the runoff so far, though only a fraction has come from Handel's campaign. She launched her first TV ad of the runoff two weeks ago, focusing on her experience in the district, and has spent or reserved about $1.7 million in ads. Most has gone to spots on broadcast TV and Fox News.

GOP groups have rushed to try to fill the void. The National Republican Congressional Committee has laid out more than $4.1 million and the Congressional Leadership Fund – which has close ties to House Speaker Paul Ryan – doled out $2.8 million. The U.S. Chamber put in about $1 million.

The handwringing among Republicans is well underway.

"It's a real problem when one campaign outspends the other about 7-1," said a senior GOP official who requested anonymity to discuss sensitive issues, "and an even bigger problem when the Handel campaign's message is, 'I'm a career politician with friends.'"

(Handel's campaign said in a statement that it will continue to highlight her "proven record of results" in more than two decades of service in public office and the private sector.

"She's going to take that experience to Washington to fix our broken tax code, balance the budget, repair our infrastructure, get our economy going, and repeal and replace Obamacare," the campaign said.)

These spending totals are likely to change dramatically. Democrats have been upping their buys in gradual increments and the candidates get lower ad rates than outside groups – which means Ossoff, who has become a fundraising dynamo, gets more bang for his buck. Analysts expect the total cost of the race to surpass $40 million.

Contrast the flood of spending in Georgia with the other recent House special elections. Montana's air wars cost about $10 million, though Democrats only reluctantly helped Rob Quist after weeks of attack ads from GOP groups. And less than $200,000 was spent by outside groups in the Kansas race.

Both those contests were fought over reliably Republican districts that Donald Trump won by 20 points or more. And in both, national Democrats were squeamish about pumping in more resources into campaigns they saw as doomed.

Georgia's 6th District, which spans from east Cobb to north DeKalb, has also long been a GOP stronghold. But Trump won the district by less than 2 points, and Democrats were buoyed by Ossoff's near-miss of an outright victory on April 18.

Democratic operatives see something more in the contest: They need to pick up two dozen House seats to take back control of the U.S. House. And the path might run through suburban districts like the 6th scattered across the South rather than trying to flip rural Republican bastions that would be harder to defend.

More recent AJC coverage on the 6th District race: