(Reuters) - Melania Trump's big night at the Republican National Convention failed to draw bumper television audiences despite networks boosting their coverage of the gathering and husband Donald Trump promising a "showbiz" feel to the week.

According to preliminary ratings data on Tuesday from the six leading broadcast and cable networks, some 22.5 million people watched Monday evening's highlight during the 10 p.m. to 11 p.m. hour when the wife of the Republican White House contender delivered her speech.

The focus of the convention shifted on Tuesday from the formal anointing of her husband to accusations that sections of her speech were strikingly similar to an address by Michelle Obama at the Democratic convention in 2008.

The early viewing figures were on a par with the audience for the first night of the Republican convention in 2012. Updated figures, along with those from smaller networks, were expected later Tuesday.

U.S. television networks have boosted coverage of the Republican convention this year after Trump, a New York businessman, drew huge audiences during the 2016 presidential debates. Trump, the former star of "The Apprentice" TV show, has promised to throw out traditional political convention scripts and "put some showbiz" into the mix.

Media analysts predicted that viewership for Trump's acceptance speech this week for the Republican presidential nomination could surpass the record 38 million who watched Barack Obama address the Democratic convention in 2008.

In 2012, the Democratic convention generally drew more nightly TV viewers than the Republican gathering, when Mitt Romney was running for the White House.

On Monday, the Fox News cable channel, as expected, got the most viewers for its convention coverage, averaging 6.4 million in the 10 p.m. hour. Among the mainstream networks, NBC led the way with 3.9 million.

However, the biggest single program audience of Monday night was for reality show "The Bachelorette," which was seen by some 6.6 million on ABC. About 3 million people turned off when ABC switched to the convention.

(Reporting by Jill Serjeant; Editing by David Gregorio and Jeffrey Benkoe)