DENVER — President Barack Obama and Republican Mitt Romney come face to face for the first time in this presidential campaign Wednesday night for a nationally televised debate that will give millions of Americans a chance to size up two fierce competitors in a moment of high-risk theater.

Romney, trailing in polls in a number of key states and running short on time to reverse his fortunes, is angling for a breakout performance in the three 90-minute presidential debates scheduled over the next three weeks.

Obama, well aware that the remaining five weeks of the race still offer enough time for tectonic shifts in his prospects, is determined to avoid any campaign-altering mistakes as he presses his case for a second term.