In a presidential election year, no glowing rectangle in Iowa or New Hampshire is safe from an endless deluge of political ads.

Campaign ads are inescapable on the nightly news, “Wheel of Fortune” and YouTube. Even the high-dollar, high-visibility ad blocks of professional and college football games have become increasingly saturated.

It’s a deeply entrenched multimillion-dollar industry, and one of the largest expenses of every presidential campaign. But a confluence of political forces and changing media behavior may be testing the efficacy of political advertising in the Trump era.

Nikki Haley and her allied super PAC spent roughly $28 million on broadcast ads in Iowa, according to AdImpact, an ad-tracking firm. Gov. Ron DeSantis and his allies spent $25 million. Trump and his super PAC spent only $15 million — and won by more than 30 points.

As my colleagues Michael Bender and Katie Glueck reported, that result showed a new depth to the Republican Party’s devotion to Trump. But it also suggests that a smaller universe of persuadable voters and a wholesale shift in viewing habits may have significantly undercut the impact of political advertising.

According to Cross Screen Media, an ad analytics firm, only 63 percent of Iowa Republicans are reachable with traditional or “linear” TV ads, as viewers switch to streaming and social media. In 2016, that percentage was still in the 90s. At most, Republican campaigns this year reached 42 percent of likely caucus voters.

“I don’t think that people have caught up with where the media consumption is,” said Michael Beach, chief executive of Cross Screen Media.

The pivot to streaming is potentially deadly for political ad buyers. Beach estimates that almost 40 percent of the time viewers spend on television is on streaming, but streaming offers far fewer opportunities to show ads to viewers than traditional programming.

New Hampshire’s presidential race is much closer than Iowa’s was, with polls showing Haley trailing Trump by single digits. And Trump faces a similar advertising deficit, with Haley and her allies spending more than twice as much as Trump’s campaign and its allied super PAC.

But the tone of advertising in New Hampshire has taken a sharply negative turn on the former president. Ten times as many negative ads attacking Trump have run in New Hampshire over the past 30 days as ran in Iowa, according to data from AdImpact. The biggest such spender is the SFA Fund, the super PAC supporting Haley, which is portraying Trump as a liar prone to temper tantrums.

Trump and his allies have responded, spending $1.4 million on a single ad attacking Haley over immigration, and $2.7 million on one targeting her support for raising the gas tax when she was governor of South Carolina in 2015 (she also called for a corresponding income tax cut).

The New Hampshire ads reveal the key issues that each campaign is hoping will boost their support in the final days. The Trump campaign and MAGA Inc., the super PAC supporting his campaign, have spent more in New Hampshire on ads regarding immigration than any other issue, according to AdImpact.

Haley’s campaign has almost exclusively run ads portraying her as representing a “new generation” and castigating Trump and President Biden as too old for the presidency. The SFA Fund has made taxes core to its ad campaign, with nearly half its ad spending over the past month promoting Haley’s pledge to cut taxes for the middle class tax or defending her record on taxes.

(DeSantis, who is far behind Trump and Haley in New Hampshire, had not broadcast any ads in the state in over a month when DeSantis and his super PAC announced Wednesday that they would be leaving the state to focus on South Carolina.)

But there maybe slightly more of an opportunity for Haley to close the gap. According to Cross Screen Media, 80 percent of New Hampshire Republican voters are reachable by traditional television advertising.

Thanks to a combination of coincidence, scandal, health issues and political turmoil, the Republican majority in the House of Representatives keeps getting smaller.

This week, with lawmakers absent for medical reasons and the recent not-so-voluntary departures of the ousted former speaker Kevin McCarthy and the expelled George Santos, the best G.O.P. attendance that Speaker Mike Johnson can muster as he tries to avoid a government shutdown is the bare-minimum 218 votes. That is before factoring in the impact of rough winter weather across the nation.

Another Republican, Representative Bill Johnson of Ohio, is resigning as of Sunday to take a job as a university president, lowering the number to 217 if Representative Harold Rogers of Kentucky, the 86-year-old dean of the House, is unable to quickly return from recuperating from a car accident. Representative Steve Scalise of Louisiana, the No. 2 Republican, is out until at least next month while undergoing cancer treatment.

As a result, the G.O.P. could soon be able to afford just a single defection on any matter if Democrats remain united and have no absences of their own.

Republicans are in a real numerical bind. At a time when House Republicans regularly face internal rebellion from hard-line conservatives, Johnson has absolutely no cushion if he chooses to rely strictly on the votes of his own party, which is part of the reason he cut a deal with Democrats on spending to avoid a shutdown later this week, further angering the hard right.

Democrats say the recurring scenario of leaning on them for must-pass bills is proof that even though Republicans are the majority party on the tally sheet, they don’t have a working majority because of their diminished forces and constant internal squabbling.

“When anything hits the fan, they don’t have 218,” said Representative Steny H. Hoyer of Maryland, the former longtime Democratic majority leader, referring to the number that represents a basic majority in the 435-member House. “They are not the majority party in this House.”

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