The tight race between Democrat Danny O’Connor and Republican Troy Balderson in a deep-red Ohio House district Tuesday reinforced a trend that has been developing for more than a year: Democrats are routinely beating their 2016 performance by double digits — putting the House and perhaps even the Senate within their grasp in November.

In Ohio’s 12th Congressional District, which encompasses the northern suburbs of Columbus and rural expanses to the east, voters preferred President Trump by 11 points and GOP Rep. Patrick J. Tiberi by 37 points in 2016. But with the race too close to call late Tuesday night, those voters showed a marked shift away from Republicans.

The war for control of Congress has been, for a year and a half, fought in a series of such skirmishes — their names ringing in the ears of Washington political operatives like famous battlefields: Georgia 6. Arizona 8. Pennsylvania 18. And Alabama — never forget Alabama.

The battle moves now to a broader tableau, away from one-offs marked by floods of outside spending and heavy national media coverage to a 435-district scramble where any one contest will have difficulty standing out.

In race after race, national Republican groups have intervened with spending to offset strong fundraising from Democratic candidates. More than $6 million was spent to benefit Balderson, according to pre-election campaign finance reports, versus the $1.2 million spent in support of O’Connor.

Trump personally endorsed the GOP special election candidates and, in some cases, held political rallies to drum up votes among supporters who might not be inclined to back a generic Republican, such as Balderson, a longtime state legislator who shied away from an enthusiastic embrace of the president.

Now, with the battlefield expanding to dozens of House districts and a handful of key Senate races, those particular advantages stand to be diluted. Instead, political strategists say, the fundamentals will become more salient.

“It makes what’s happening organically more important,” said Zac McCrary, a Democratic pollster who worked on special elections in Alabama and Georgia and is advising numerous midterm campaigns. “There’s a Democratic intensity advantage. Organically, independent voters are tilting Democratic in most places. . . . Everything else being equal, the playing field advantages the Democrats.”

Democrats need to win 23 Republican-held seats to claim the House majority. A net gain of two seats will flip the Senate, but many more Democrats are vulnerable to Republican challengers this year.

Republican campaign officials acknowledge the head winds but say they are confident that a strong economy, smart campaigns and targeted spending by national GOP groups will mitigate the Democrats’ advantage. They point to an overall 7-2 record in the special elections for GOP-held congressional seats before Tuesday.

President Trump, right, stands beside Republican House candidate Troy Balderson, left, during a rally Saturday in Lewis Center, Ohio. (John Minchillo/AP)

But that tally obscures the larger trend. In an April 2017 Kansas race, Republican Ron Estes beat Democrat James Thompson, but only by six points in a district Trump won by 27. Two months later, Republican Ralph Norman won a South Carolina seat — by three points in a district Trump won by 19. In an Alabama shocker, Doug Jones became the first Democrat to represent his state in the Senate in two decades. And in March, Democrat Conor Lamb broke through in western Pennsylvania, eking out a 755-vote victory in a district Trump won by 20 points.

What has been more telling has been the relative stability of the Democratic campaign message. Virtually every Democratic special-election candidate has run on health care and economic fairness — not taking direct aim at Trump and his administration as much as a Republican policy agenda that they say favors the rich and well-connected over ordinary Americans.

Republicans, meanwhile, have flitted from issue to issue seeking to promote their own candidates and disqualify Democrats. Individual candidates have been at sea trying to focus on local issues while outside groups prosecute national lines of attack on their Democratic opponents.

Early hopes of riding last year’s GOP tax cuts to victory have largely faded along with the tax bill’s popularity; more recently, Republicans have seized on calls by some liberal Democrats to abolish the Immigration and Customs Enforcement agency as fodder for attacks.

But for candidates such as Lamb and O’Connor, the GOP has had a hard time making the charge of radicalism stick. More frequently, the play has been guilt by association — specifically, association with House Minority Leader Nancy Pelosi (D-Calif.), who has near-universal name recognition among voters.

Ads attacking Pelosi’s “San Francisco values” derailed Democrat Jon Ossoff’s bid for a suburban Atlanta seat, and O’Connor was tripped up last month after suggesting, under persistent questioning by MSNBC’s Chris Matthews, that he could end up voting for Pelosi as speaker after saying that he wouldn’t.

O’Connor also faced ads accusing him of being soft on illegal immigration and supporting cuts to Medicare. “Message whiplash,” O’Connor pollster Jason McGrath called it Tuesday before polls closed.

If Balderson ultimately prevails, it is Trump, ironically, who may deserve the lion’s share of the credit for pushing him over the top. At a Saturday rally in suburban Delaware County, the president delivered effusive praise for Balderson and mocked “Danny Boy,” predicting a “red wave” that would wash over the nation come November.

“But you got to get out and do it,” Trump said. “You got to get out and vote. You got to get out because they want to take away what we’ve given. . . . A vote for Danny Boy and the Democrats is a vote to let criminals and drugs pour into our country and to let MS-13 run wild in our communities.”

Republicans will be hard-pressed to replicate the one-two punch they landed on O’Connor. Not only did Trump stump for Balderson, but Gov. John Kasich, a Republican moderate who once held the seat, cut a late TV ad on Balderson’s behalf.

Democrats are taking heart that O’Connor’s strong showing reflected a message carrying the same ring as the party’s candidates in suburban and blue-collar districts across the country — heavy on bipartisanship and “kitchen table” economic issues.

“Pennsylvania 18 looks a lot different from Ohio 12, and yet you are seeing very similar messages work,” said Tyler Law, a spokesman for the Democratic Congressional Campaign Committee. “What we’re learning is that message is going to resonate with a lot of different people.”

One Republican strategist also warned that the tight Ohio race was the latest wake-up call for GOP candidates who have lagged in fundraising, hoping national groups would pick up the slack. Those groups are going to be stretched thin over the next 90 days, defending a House battlefield stretching from Bangor, Maine, to California’s Orange County.

“If you are getting outraised, you are asking for trouble,” said Corry Bliss, executive director of the Congressional Leadership Fund, which spent more than $3 million on TV ads and door-knocking in Ohio. “You are asking to lose.”

And while Trump may be a secret weapon in some of those districts, Democrats say that is not true everywhere. Many of the party’s top targets are in suburban districts where Trump is seen as a drag on Republicans more than a boost for them. And in the rest, there’s only so much one man can do — even the president.

“It is clear that Republicans think their best mechanism here to try to salvage these districts is sending in Trump a day or two before,” McCrary said. “You know, Donald Trump can’t be in 40 districts in the last 72 hours before the November election.”