John McCain is expected to reveal his vice-presidential choice on Friday at a rally in the critical battleground state of Ohio.

Mr McCain is seeking to blunt Barack Obama’s hopes of a bounce in the polls after he accepts the Democratic nomination in Denver tomorrow night.

The Republican nominee’s move comes after a week in which he and his Republican surrogates have had significant success in inserting themselves aggressively into what was meant to be Mr Obama’s week. Republican guerrilla tactics and attack advertisements have forced the Democrat’s campaign to respond in kind.

Mr McCain will appear at a midday rally in Dayton before his own nominating convention opens in Minneapolis on Monday. He will then tour three of the most important battleground states — Ohio, Pennsylvania and Missouri — with his newly announced running-mate this weekend.

Just as the choice of Joe Biden last week as the Democratic vice-presiden-tial nominee was a well-kept secret, the identity of Mr McCain’s pick has also been guarded closely.

Speculation has focused on a handful of possible candidates. They include Mitt Romney, the former Massa-chusetts Governor, who was defeated in the hard-fought primary campaign by Mr McCain; Tim Pawlenty, the 47-year-old Minnesota Governor; Joe Lieberman, Al Gore’s 2000 running-mate who has become a staunch ally and close friend of the Republican nominee over the issue of Iraq; and Kay Bailey Hutchison, the first woman senator from Texas.

Mr Obama accepts his party’s nomination tomorrow night after a week of unremitting negative advertisements by both sides and a declaration of intent from the Democrat that he is not going to allow any Republican attack to go unanswered. It is a sign of how intense and increasingly toxic the race has become between the two, who pledged to rise above such hardball tactics.

Aware of how John Kerry’s failure to rebut the “swift-boat” attacks against him quickly in 2004 damaged his presidential campaign, Mr Obama has used negative advertisements in a series of swing states. He also made an aggressive counterattack against a Republican commercial linking him to William Ayers, a former terrorist.

The McCain campaign set up a war room in Denver for the Democratic convention, with figures such as Mr Romney giving interviews and holding press conferences criticising Mr Obama. Such tactics, combined with the daily cycle of negative commercials, are virtually unprecedented.

This year’s equivalent of the swift-boat campaign was started on Monday by a Republican group called the American Issues Project, with an advertisement that says Mr Obama has defended Mr Ayers, and that he started his political career from his house. He is a former member of the Weather Underground, a radical group that bombed government buildings in the 1970s.

“Why would Barack Obama be friends with somebody who bombed the Capitol and is proud of it?” a narrator asks. “Do you know enough to elect Barack Obama?” The commercial is financed by Harold Simmons, the Texas billionaire who funded the 2004 swift-boat campaign.

The Obama campaign responded immediately with an advertisement to counter the Ayers commercial, while his campaign lawyer has written to television stations saying it is illegal and false, and threatening legal action.

A Republican official called such tactics intimidation. Robert Bauer, the Obama lawyer, said: “If someone rides up to a convenience store with a sawed-off shotgun and a prior record, I’m not intimidating anybody by calling the cops.”

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