DENVER (AP) — Primed for a showdown, President Barack Obama and Republican challenger Mitt Romney left their practice sessions behind Wednesday for a prime-time debate before millions with the power to settle the race for the White House in tough economic times. The encounter at the University of Denver was the first of three for the candidates. By agreement between the rival campaigns, it was focused on the economy and other domestic issues. Jim Lehrer of PBS drew moderator’s duties. Obama was getting the first question, Romney the last word. Romney had a private tour of the debate hall Wednesday afternoon. Obama planned a walk-through after arriving from Nevada, where he spent three days in practice sessions. Romney’s aides said he reviewed briefing books and policy at his Denver hotel Five weeks before Election Day, early voting is under way in scattered states and beginning in more every day. Opinion polls show Obama with an advantage nationally and in most if not all of the battleground states where the race is most likely to be decided. That put particular pressure on Romney to come up with a showing strong enough to alter the course of the campaign. The sputtering economy served as the debate backdrop, as it has for virtually everything else in the 2012 campaign for the White House. Obama took office in the shadow of an economic crisis but promised a turnaround that hasn’t materialized. Economic growth has been sluggish throughout his term, with unemployment above 8 percent since before he took office. Both campaigns engaged in a vigorous pre-debate competition to set expectations, each side suggesting the other had built-in advantages. Romney took part in 19 debates during the campaign for the Republican primary early in the year. The president has not been onstage with a political opponent since his last face-to-face encounter with Arizona Sen. John McCain, his Republican rival in 2008. Obama and Romney prepared for the evening with lengthy practice sessions. Romney selected Ohio Sen. Rob Portman as a stand-in for the president; Obama turned to Massachusetts Sen. John Kerry to play the Republican role. The two presidential rivals also are scheduled to debate on Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla. Vice President Joe Biden and Republican Rep. Paul Ryan of Wisconsin have one debate, Oct. 11 in Danville, Ky.