U.S. President Barack Obama plans to tell voters they “face the clearest choice of any time in a generation” when they go to the polls this November, asking them to choose between two different paths for America.

In excerpts released in advance of Mr. Obama's acceptance speech , Mr. Obama says “I won't pretend the path I'm offering is quick or easy…The path we offer may be harder, but it leads to a better place.”

The president will also lay out specific goals for the U.S. economy, calling for one million new manufacturing jobs by the end of 2016, doubling exports by the end of 2014 and cutting net oil imports in half by 2020. He will also pledge to cut the nation's deficit by more than $4 trillion over the next ten years.

Mr. Obama's speech comes just nine weeks ahead of the November 6 general election. Mr. Obama and Republican challenger Mitt Romney are in a virtual tie in opinion polls.

Earlier Thursday, the Democrats formally nominated Mr. Obama's running mate, Joe Biden, for vice president.

Like the president, Biden will speak at the convention later Thursday. He was nominated by his son, Beau, who called him a hero. Images of the vice president showed him tearing up as his son spoke.

Earlier Thursday, Mr. Obama held a conference call with supporters who were supposed to see him in person as he delivered his acceptance speech.

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

Mr. Obama said during Thursday's conference call that the move was “disappointing” but he could not put the safety of supporters and volunteers at risk.

He also told supporters he expected a close race against Republican challenger Mitt Romney. He said Mr. Romney's campaign was preparing to release a “barrage” of negative ads.