Barack Obama, hoping to inject new energy into the Democratic convention, will break with tradition and deliver his first speech as the party's presidential nominee before an audience of 75,000 people in Denver. The party said yesterday it would open up the convention's pivotal moment to public spectators.

The move was intended to symbolise Obama's promise of a transformational presidency, as well as the historic aspect of his candidacy as the first African-American to lead a major party into a US election, the party said.

"The Democratic party is nominating a true change candidate this August, and it is only fitting that we make some big changes in how we put on the convention," Howard Dean, the party's national committee chairman, said. "By bringing the last night of the convention out to the people we will be able to showcase Barack Obama's positive, people-centred vision for our country."

The plan also aims to capitalise on Obama's talents as an orator who can excite large audiences, unlike the Republican candidate, John McCain.

Invesco Field, the outdoor stadium in Denver where Obama will deliver his acceptance speech on the last day of the convention, has more than three times the capacity of the centre where Democrats will hold their meeting at the end of August. That venue holds about 20,000.

Obama's speech on August 28 will be on the 45th anniversary of Martin Luther King's historic "I Have a Dream" address in Washington.

After a week devoted to themes of God and country, Obama and McCain yesterday shifted the focus of their campaigns to the more immediate issues of the economic and energy crises. McCain led off his economic campaign with a promise to cut the record deficits of the Bush administration to produce a balanced budget by 2013. Current projections suggest there will be a $443bn (£224bn) deficit in 2013, fuelled by spending on the Iraq war and Bush's tax cuts. McCain aides sharpened their attacks on Obama as a conventional liberal who would raise taxes.

In a conference call with reporters, Steve Forbes, the onetime independent candidate who is serving as a McCain economic adviser, said Obama's policies would devastate the economy. He also accused Obama of changing positions. "This gets to the whole credibility problem with Senator Obama saying one thing and then doing another," he said.

McCain, in a visit to Denver, took Obama to task for his opposition to the Republican's proposal for a summer reprieve on petrol taxes. "For a guy whose official seal carried the motto 'Yes, we can', Senator Obama's agenda sure has a whole lot of 'No, we can't'," McCain said.

Obama yesterday outlined a $50bn economic stimulus package, which he said would provide some security to middle income families, during an unscheduled stop in St Louis, Missouri. He had been scheduled to travel to North Carolina, but was diverted to St Louis by technical difficulties.

In his speech, Obama accused McCain of favouring big oil companies and multi-millionaires with $300bn in tax cuts. "If this sounds familiar, it's because it's exactly what George Bush has done for the last eight years," Obama said.