AT&T is the second-largest mobile network operator in the US and the third-largest cable TV provider, while Time Warner controls a valuable share of entertainment content suppliers.

The $85.4bn (£69.8bn) takeover will, if approved by regulators, give AT&T control of cable TV channels HBO and CNN, film studio Warner Bros, the TNT and TBS cable channels, and other coveted media assets.

AT&T will have control of a massive catalogue of popular content from sports to films and TV series including Game Of Thrones, The Wire, Sex And The City and The Sopranos.

Both presidential candidates have shared concerns about the deal, but those behind the bid are probably banking on an easier ride if Hillary Clinton is elected on 8 November.

Republican Donald Trump, who has already railed against the media's role in what he has described as a "rigged" election, has said he will block it if he is elected.

Mr Trump's economic adviser Peter Navarro said: "Donald Trump would never approve such a deal because it concentrates too much power in the hands of the too powerful few."

Mrs Clinton' spokesman, Brian Fallon, has expressed caution, saying there was "still a lot of information that needs to come out before any conclusions should be reajched".

The Senate subcommittee on antitrust will meet in November to hold a hearing on the deal.

The power to block it lies with the US Justice Department, not the president, and AT&T said it is unsure as to whether the Federal Communications Commission also has jurisdiction.

US Senator Richard Blumenthal, a member of the Senate Judiciary Committee, said: "Such a massive consolidation in this industry requires rigorous evaluation and serious scrutiny.

"I will be looking closely at what this merger means for consumers and their pocketbooks."

Senator Al Franken, also on the Judiciary Committee, said the deal raised "immediate flags".

"I'm sceptical of huge media mergers because they can lead to higher costs, fewer choices, and even worse service for consumers," he said.

AT&T has said the merger is "vertical", as there is no overlap between the two companies. It hopes the deal will be concluded by the end of next year.

"There's no competitor being removed from the marketplace, there's no competitive harm that is being rendered by putting these two companies together," said AT&T CEO Randall Stephenson. 

"So any concerns by the regulators, we believe, will be adequately addressed by conditions - that's our anticipation."

The five biggest mergers in history:

:: $180bn - Vodafone (UK) and Mannesmann AG (Germany) - 1999

:: $165bn - Time Warner (US) and America Online (US) - 2000

:: $128bn - Verizon Communications (US) and Verizon Wireless Inc (US) - 2013

:: $98bn - RFS Holdings BV (Netherlands) and ABN-AMRO Holding NV (Netherlands)

:: $90bn - AB InBev (Belgium) and SABMiller (UK) - 2015