A series of outside groups and committees, viewing the race as one of the last referendums on President Donald Trump before November's midterms, have steadily poured money into the special election in Ohio's 12th Congressional District, ratcheting up the national implications in the race between Democrat Danny O'Connor and Republican Troy Balderson.

The National Republican Congressional Committee increased its spending on television ads by $250,000 on Monday, according to committee sources, reserving ad space for the final two weeks of the campaign. The committee has thus far spent close to $600,000 on the race, according to FEC reports.

The NRCC move comes days after the Democratic Congressional Campaign Committee dropped $238,000 for 10 days of ads through July 30. The expectation is that the DCCC will buy more airtime to run pro-O'Connor spots through the August 7 election.

The most constant spending in the race has come from the Congressional Leadership Fund, the super PAC backed by House Speaker Paul Ryan, which has spent close to $2 million on ads slamming O'Connor.

And Trump even weighed into the race recently, tweeting his endorsement of Balderson.

"Troy Balderson of Ohio is running for Congress against a Nancy Pelosi Liberal who is WEAK on Crime & Borders," he wrote. "Troy is the total opposite, and loves our Military, Vets & 2nd Amendment. EARLY VOTING just started with Election Day on August 7th. Troy has my Full & Total Endorsement!"

More than $2 million worth of TV ads have been dumped on the district by campaigns and outside spending groups, according to data from the ad-tracking firm CMAG, and those outside groups have combined to spend more than $4 million in total on the special election, per FEC filings.

A test run for November

The uptick in money and attention is the latest sign that both Democrats and Republicans are closely watching the special election as a sign of how each party will fare in November. The congressional district, whose seat was opened when Rep. Pat Tiberi retired, has long been represented by Republicans and in another political environment would likely be out of reach for most Democrats.

What has surprised Democrats most in the race, however, is the fact that Republicans have largely abandoned the pro-tax reform messaging that national operatives called central to their 2018 strategy just months ago.

"There is no positive outcome in November if we do not show that we cut taxes for the middle class and are working to make their lives better. Period," Congressional Leadership Fund executive director Corry Bliss wrote in a memo earlier this year.

That strategy has not played out in the special election, however, where the Ryan-aligned outside group has spent much of its time on air linking O'Connor with House Minority Leader Nancy Pelosi and Massachusetts Sen. Elizabeth Warren.

CMAG has found that taxes are not the top issue on TV, either. According to the group, taxes have been the second most talked about issue on television in the race, behind immigration. But health care is a close third, followed by government spending and social issues.

"The liberal resistance ... want Danny O'Connor's help, bankrolling his campaign. Pelosi and Warren know O'Connor supports amnesty for illegals and O'Connor opposes the border wall," says a narrator in a recent CLF ad. "Danny O'Connor would join the resistance."

At no point does the ad mention the tax bill, a fact that surprised Democratic operatives working on the race.

"After selling their conference on the GOP tax plan, this race has once again shown it may in fact be more of a liability than an asset," said the strategist. "This district is also majority suburban and has the highest median income in the state, begging the question: If you can't sell the tax bill here, where can you?"

Courtney Alexander, a spokeswoman for CLF, argued that the group had made tax reform part of their messaging -- noting two ads earlier in the campaign that mentioned the issue.

"The Democratic Party has become the party of mocking middle-class families' tax cuts, calling for higher taxes, and now, abolishing ICE," she said. "That's a nice contrast with the Republican Party, who voted to cut middle-class taxes."

Litigating tax reform

At the same time, Democrats have increased their focus on the tax bill, which Republicans -- with Trump's backing -- passed earlier this year.

After showing people around Balderson smiling and laughing, a narrator in the latest DCCC asks why they all look so happy.

"It's because Balderson supports a massive corporate tax break that helps rack up $2 trillion in debt," the narrator says, going on to argue that the tax bill could mean cuts to Medicare and Social Security. "Troy Balderson, that's not funny at all," the add concludes, showing a laughing Balderson.

O'Connor has also looked to fight being linked to national Democrats, eschewing the fiery rhetoric and leftward swing dominating the Democratic Party to reach out to longtime Republican voters. O'Connor blunted Republican attacks against him when he pledged to vote against Pelosi and has run as a pragmatic leader willing to work with Republicans in Congress and Ohio.

In a recent ad, O'Connor's campaign touts his ties to Ohio Gov. John Kasich, a controversial figure among Republicans in Ohio because of his opposition to Trump. Kasich has yet to endorse Balderson, even though the governor once represented the congressional district.

"I voted for John Kasich the last three times. I voted for Trump because I didn't like the way things were going in Washington, and now I'm supporting Danny O'Connor," Shannon Ward, a 40-year old mother of two from Delaware, Ohio, says in the ad.

Democrats in Washington and Ohio believe that if they are able to win the special election on August 7, their chances of taking back the House in November are high. Republicans, eager to defend Trump's presidency, want to put a stop to a string of strong performances by Democrats in special elections.