The majority of ads aired by Republicans this year? They're on the attack.

The analysis, which examined broadcast television advertising month-by-month, shows a sharp rise in the negative ads as the midterms drew closer, jumping from 43% of Republican ads in July to 53% in August and 59% in September once the general-election season kicked into high gear.

The stakes are high, particularly in the House, where Democrats need to win just 23 GOP-held seats to seize the majority. Democrats are competing fiercely in districts across the map. For their part, Republicans are contending with President Donald Trump's low approval ratings in some key districts and the weight of history: A first-term president's party typically loses ground in midterm elections.

Leading the way in attack ads for the GOP: The Congressional Leadership Fund, a deep-pocketed super PAC aligned with House Speaker Paul Ryan, a Republican from Wisconsin.

The group, also known as CLF, was responsible for nearly one-fifth of the roughly 101,000 negative congressional ads Republicans aired in September, according to the CNN examination of data from Kantar's Campaign Media Analysis Group, which tracks political advertising. The review counts the number of times a commercial aired, not the number of unique ads unveiled by candidates or groups.

"Negative" ads, as determined by Kantar, are spots that paint a candidate in an unflattering light. Ads that attack an opponent but don't mention the candidate being supported are typically classified as negative.

In its hard-hitting advertising campaign, CLF has sought to tie Democrats to Islamic terrorism and branded one House candidate, a Harvard-educated lawyer and former rapper, as too "radical and extreme" for the Hudson Valley, New York, district he's running to represent.

"I think the CLF believed that it was important to go up earlier and try to define these Democrats before they were able to get too much momentum, and it looks like they decided to leave subtlety behind and just go full force," said Nathan Gonzales, editor and publisher of the non-partisan Inside Elections political newsletter.

The advertising themes also show Republicans believe "they still have the upper hand when it comes to issues of national security and immigration," he said.

Officials with the Congressional Leadership Fund say their job is to confront Democratic candidates.

"In a challenging environment, CLF has successfully been able to define Democrats and put first-time candidates on defense," said Courtney Alexander, a spokeswoman for the super PAC. "We've been able to set the terms of these races across the country, and Democrats have been forced to scramble to answer for their records as the truth comes out."

Democrats, led by the Senate-focused Majority Forward group, also are on the attack in their advertising, totaling more than 96,000 spots last month. But negative ads account for just 36% of the spots the party, its candidates and outside groups aligned with Democrats ran in September, according to Kantar's data.

Ads spark controversy

One of the most hotly contested races -- the battle between first-term GOP Rep. John Faso and lawyer Antonio Delgado in upstate New York -- also has seen some of the most controversial ads of the general-election season.

One recent CLF ad used Delgado's brief, decade-old hip-hop career as "AD the Voice" to paint him as "radical and extreme" and dangerously out of touch with the district. Delgado is African-American, and the district is predominantly white.

The National Republican Congressional Committee also seized on that message, recently running a 30-second spot depicting Delgado as an un-American misogynist. His music included lyrics about sex and criticized the "white supremacy" of "dead presidents."

Delgado, a Rhodes Scholar who worked at the large Akin Gump law firm, has accused Republicans of attempting to "otherize" him with the advertising.

"The truth is I grew up right here in Schenectady, where I learned the values of hard work and accountability," Delgado said in a statement.

Republicans have not backed away from the message.

In an interview with the CBS television affiliate in Albany, Faso declined to denounce the ads.

"These are his words," Faso said of the Delgado lyrics. "But those are ads that are done independently of my campaign and I have no control or input into them,"

Several other GOP-funded ads in tight races have raised the specter of undue influence by Islam should Democrats capture new seats or retain the ones they hold.

In another too-close-to-call House race, a recent CLF ad criticized Virginia Democratic candidate Abigail Spanberger for her substitute-teaching job at a Saudi-funded Islamic school in Virginia. The group and other critics have dubbed the school "Terror High" because some graduates later joined al Qaeda.

Spanberger, a former CIA officer, is trying to oust Republican Rep. David Brat from his Richmond-area House seat.

Different messages

The review of advertising shows Democrats and Republicans taking a sharply different approaches in the issues they tackle on the airwaves.

Half of the television spots run by Democrats this year focus on health care, many arguing that Republican policies will endanger the health care of Americans with pre-existing conditions.

Republicans, however, have not settled on a single theme.

About 30% of GOP-aligned ads this year focus on taxes, but issues important to the Republican voter base, including support for Trump and anti-immigration messages are close behind.

Despite strong economic news, jobs and the economy trail other hot-button topics in Republican ads.

"It always gets more negative at this time of year, but Republicans don't have a great reliable issue to talk about right now," said Madeline Meininger, who analyzes advertising at Kantar/CMAG.

Veteran Republican pollster Whit Ayres said voters tend to focus on the negative when elections roll around.

"It's hard to run on a booming economy if that's not a problem people are facing," he said.

"Everyone knows that midterm elections are tough for the party in power," Ayres added, "so it makes sense that Republicans are raising fears of what the government would be like if Democrats take control."