U.S. President Barack Obama will use Thursday's acceptance speech at the Democratic National Convention to spell out his economic vision for the country and attempt to woo undecided voters.

Mr. Obama's adviser David Plouffe told ABC television Thursday that, after the address, voters would have a “very clear sense” of where the president believes the country should go economically.

Nine weeks ahead of the November 6 general election, Mr. Obama and Republican challenger Mitt Romney are in a virtual tie in opinion polls.

Plouffe said he did not expect the Democratic convention to produce a “big bounce” in the polls. In the TV interview, Plouffe said he expected the race to remain tight until the end.

Ahead of his address, Mr. Obama is holding a conference call with supporters who had hoped to see him in person as he delivered his acceptance speech.

Convention organizers had planned to stage Mr. Obama's speech at a 74,000-seat outdoor football stadium in Charlotte, North Carolina. They scrubbed those plans Wednesday and moved the speeches to a much smaller indoor arena, citing the threat of thunderstorms.

Republican critics said the Obama campaign's real reason for the switch was a fear of having empty seats at the huge outdoor stadium.

President Obama's campaign received a major boost Wednesday from a speech by former President Bill Clinton, who remains a popular figure among many Americans who recall economic prosperity during his two terms in office in the 1990s.

Mr. Clinton offered a strong defense of the current president's economic record against Republican claims that conditions have worsened since Mr. Obama took office in 2009.

“When President Barack Obama took office, the economy was in free fall. It had just shrunk nine full percent of GDP. We were losing 750,000 jobs a month. Are we doing better now today? The answer is YES.”

Mr. Obama walked on stage after Mr. Clinton's speech, and the two men enjoyed a warm handshake and embrace.