Good news for the cable-news business: Republican presidential nominee Donald Trump and Democratic nominee Hillary Clinton face off during the second presidential debate at Washington University in St. Louis. (John Locher/AP)

Donald Trump and Hillary Clinton have been very, very good for the cable news networks.

Thanks to an audience surge driven by nearly round-the-clock campaign coverage, the leading news networks will reap unpre­cedented profits this year.

According to people intimately familiar with CNN’s finances, the network and its related media businesses will approach $1 billion in gross profit in 2016, a milestone unseen in its 36-year history. The internal estimate reflects a double-digit increase over 2015 and includes CNN’s international network, its popular website and the smaller HLN network, but it is driven primarily by its domestic channel, according to people at CNN.

Fox News, the longtime ratings leader among cable-news outlets, was already in the billion-dollar profit club before 2016. Despite a tempestuous patch this summer, when its parent company ousted Fox’s chairman and co-founder, Roger Ailes, Fox News is projected to have its most financially lucrative year: Its gross profit will top $1.67 billion, according to SNL Kagan, a media research company.

Third-ranked MSNBC, which falls under the larger NBC News umbrella, will have projected earnings of $279.6 million this year, according to Kagan. MSNBC is growing faster in percentage terms than its two much-larger competitors: Its projected profits reflect a 19 percent growth rate over 2015, compared with 14 percent for CNN and 11.3 percent for Fox, according to Kagan.

The common thread for all three organizations has been improved ratings because of the presidential campaign, said Scott Robson, a research analyst at Kagan.

Trump, in particular, seems to have been a magnet for attracting viewers. The first Republican debate, in August of 2015, carried by Fox, attracted 24 million viewers, the most ever for an event during the primaries. That was bookended by the first presidential debate featuring Trump and Clinton in late September, which attracted 84 million viewers, a record audience for such an event.

In between, the networks have larded their airwaves with copious coverage of Trump’s rallies and events, leading to criticism that all the attention propelled Trump to the Republican nomination. Jeff Zucker, president of CNN Worldwide, recently acknowledged that at least some of CNN’s coverage was excessive, but he denied that it vaulted Trump to the nomination.

People have stayed tuned to cable’s coverage of more debates, candidate “town hall” meetings, campaign-trail news coverage and the ever-present panel discussions among pundits and hosts.

So far this year, all three networks have seen double-digit gains in their audiences, with each reporting all-time highs.

Fox’s daily audience has averaged 1.32 million viewers, 11 percent more than throughout 2009, its previous best year, and 15 percent ahead of the last campaign year in 2012, according to Nielsen data.

The 2016 figure has made Fox the top-rated basic cable network of any kind, a first in its 20-year history. The network still easily beats its news competitors, both among all viewers and the 25- to 54-year-olds who are the prime target of TV-news advertisers.

However, CNN said it has narrowed the audience gap with Fox to its closest margin in eight years. It has attracted an average of 748,000 viewers on a daily basis this year, Nielsen said.

The election surge has also lifted MSNBC, which jettisoned its liberal talk-show format during daytime hours last year to put a greater emphasis on campaign news. The network calls itself the fastest-growing of the three, although it is rebounding from unusually low ratings last year. To date this year, MSNBC has averaged 597,000 viewers per day, Nielsen said.

The brimming profits at all three networks validate their strategy of concentrating on political coverage nearly to the exclusion of other stories, starting in 2015. Many news organizations began expanding their political-reporting staffs last year, even as they were laying off journalists who cover other subjects.

The financial bonanza also puts Fox News’s current negotiations with prime-time star Megyn Kelly into some perspective. The Wall Street Journal reported late Wednesday that Kelly — whose profile was raised by her clash with Trump at the first Republican candidates’ debate and his subsequent criticism of her — is seeking more than $20 million a year in salary in contract-renewal talks, a raise of more than $5 million. CNN is among the networks that reportedly would like to lure Kelly, whose contract at Fox expires next summer.