BOSTON (AP) _ John Kerry toured the convention stage hours before the biggest speech of his political career, looking out over a vast expanse of empty seats and saying, ``This is great. Can we do it now?''

``Great hall,'' the Democratic presidential candidate said.

Wearing a sport coat and open-collar shirt, a beaming Kerry then gestured to the empty seats and pretended to speak, joking with a crowd of reporters gathered in front of the stage.

``Members of the Fourth Estate,'' he said to the media, ``I called you here today to tell you your reign is over,'' Kerry said as technicians checked his voice levels in preparation for his prime-time address to accept the Democratic presidential nomination.

Aides said the speech would last 50 minutes or so and would include some contrasts with President Bush, part of a strategy imposed on every convention speaker by Kerry's team. Speakers were asked to go easy on Bush-bashing and to ensure that any criticism contrasted Bush's record with Kerry's plans.

Kerry's team noted that Al Gore gave a 51-minute address to accept the 2000 nomination. Bill Clinton's 1996 speech lasted 66 minutes, and he spoke for 54 minutes in 1992. Aides said the length of Kerry's address will fall somewhere between Gore and Clinton.

After his appearance at the podium, Kerry retreated backstage for a trial run of his speech. He planned an afternoon bicycle ride before returning to the FleetCenter later in the day.