WASHINGTON — The last four national elections have been dominated by discussion of the Affordable Care Act, also known as Obamacare, and particularly by Republicans’ attacks on it. But a funny thing has happened on the way to Election Day 2018: The ACA has gone missing and health care has become a Democratic issue in campaign ads this midterm season.

The numbers tell the story of how health care has morphed into something new this election cycle, a Republican liability centered on the topic of pre-existing conditions and health conditions that predate someone’s insurance coverage.

An analysis by the Wesleyan Media Project found that in early September nearly half of all Democratic TV ads in House races, 44 percent, discussed health care in some way. It was the number one issue topic in those races. That was true of only 34 percent of House Republican races.

In Senate races the divide was even starker. There, health care was also number one topic, with 50 percent of all the TV ads in Democratic campaigns addressing it, according to the Wesleyan data. For Republican Senate campaigns, the number didn’t even crack the top five issues.

A close look at some key Senate races, using Advertising Analytics, shows how these numbers are playing on the ground. You can see the story in Arizona and West Virginia — states on opposite sides of the country, with very different races.

In Arizona, Democrats are trying to flip the Republican seat currently held by retiring Sen. Jeff Flake and turning to health care.

Since Sept. 1, 12 ads have aired in the Arizona Senate race that touched on health care. In 11 of those ads there was a direct mention of pre-existing conditions and 10 of them supported the Democratic candidate, Kyrsten Sinema. The primary claim being that her Republican opponent, Rep. Martha McSally, wants to deny coverage of the health problems.

At the same time, the one pro-Republican healthcare ad in that time attempted to rebut Sinema’s claims, arguing that McSally has voted to preserve the coverage of pre-existing conditions.

About 2,000 miles away in West Virginia, a state President Donald Trump won by 40 points, incumbent Democratic Sen. Joe Manchin is trying to hold onto his seat with a similar approach.

That race has seen 10 ads in that race on health care since Sept. 1. Eight of those ads mention pre-existing conditions and all of them support Manchin and/or attack his Republican opponent Attorney General Patrick Morrisey.

In fact, pre-existing conditions are at the center of Manchin’s favorite campaign ad device, shooting stacks of paper he doesn’t like with a rifle. In 2012, Manchin made an ad that centered on him putting a bullet through the “Cap and Trade” that he said would hurt the coal industry. This fall he is shooting “Patrick Morrisey’s lawsuit to take healthcare from those with pre-existing conditions.”

And those states are just two examples. Dig into the ad data across the country and you’ll see similar approaches in House and Senate races with by Democrats thrusting the issue out and Republicans attempting to parry.

All the back-and-forth over pre-existing conditions, has fundamentally changed the way the parties talk about the Affordable Care Act. The health insurance law that the GOP labeled Obamacare, was a favorite target of the GOP in the 2010 and 2014 midterms (and the presidential races in 2012 and 2016), but it has waned this cycle, according to an analysis this week from the Wall Street Journal.

Between January and September of 2010 and 2014, Republicans and those who support them ran more than 10 times the number of anti-ACA ads as Democrats and their supporters ran pro-ACA ads. This year the script is flipped. In 2018, Democrats and their supporters are running more than twice as many pro-ACA ads as Republicans are running against Obamacare.

And when you explore the ads, the ACA has largely become an afterthought. Democrats want to talk about covering those with pre-existing conditions and Republicans are running against the threat of single-payer health care.

The numbers serve as a reminder of two important points, not only for 2018, but for 2020 and beyond.

First, despite a political environment that’s focused heavily on the partisan debate inside Washington — everything from Justice Brett Kavanaugh to the Russia investigation — outside of D.C. candidates and voters are focused on bread-and-butter issues.

And second, the frame around an issue can change quickly. Just because one party owns an issue or topic this fall may not mean much for the campaigns ahead.