WASHINGTON (Reuters) - More than 99% of U.S. President Donald Trump’s television ads have focused on impeachment this year, according to an analysis that shows how central the topic has become for his re-election campaign.

The anti-impeachment blitz has cost the Trump campaign over $4.4 million for more than 4,500 television ad airings through Dec. 14, the Wesleyan Media Project, a nonpartisan group that tracks political advertising, said in a report on Thursday.

All of the ads mentioning impeachment have come since Oct. 1.

The House of Representatives impeached Trump on Wednesday on charges of abuse of power and obstruction of Congress - a historic vote that makes Trump only the third U.S. president to face a Senate trial to remove him from office.

The Republican-controlled Senate is expected to acquit the president. But Trump, who denies any wrongdoing, has seized on the topic as a rallying cry for his supporters, which Republicans hope will help him win re-election in November 2020 presidential contest.

Before Trump’s anti-impeachment blitz began in October, the president had only aired 36 television ads in 2019, with none mentioning impeachment, the Wesleyan Media Group said.

In addition to Trump’s ads, Republican groups are funding ads attacking Democratic members of the House over their support for the impeachment investigation.

The conservative American Action Network, for example, has spent more than $2.3 million this year on television ads that mentioned impeachment and a House candidate up for re-election in 2020.

“Republicans are going on offense when it comes to the issue of impeachment,” said Travis Ridout, co-director of the Wesleyan Media Project.

Ads by Democrats have been mostly silent on the issue, Ridout said. Almost all Democratic ads mentioning impeachment have come from billionaire businessman Tom Steyer, a presidential candidate who has made impeachment a core issue of his campaign, but whose candidacy has garnered little support in public opinion polls.