WASHINGTON  The ethics trial of Senator Ted Stevens heads to what should be a dramatic climax on Monday: a courtroom confrontation between Mr. Stevens and a Justice Department prosecutor that features a crucial telephone conversation.

Over the last three weeks, a federal jury has heard testimony and seen evidence from both sides. But all of that could become secondary to what the jurors make of Mr. Stevens, who has made a calculated gamble in taking the stand as the trial’s final witness.

Mr. Stevens, Republican of Alaska, is charged in seven felony counts with deliberately concealing $250,000 in gifts and services from a once-close friend, Bill Allen, to renovate his house in Girdwood, Alaska. The government has asserted that Mr. Allen deployed his oil services company, Veco, to help remake the Stevens home from a modest A-frame cabin to a two-story residence with wraparound porches, a garage and amenities like an expensive gas grill, a workshop and a whirlpool.

Mr. Stevens, 84, spent Friday on the stand laying out his defense: he was unaware that Veco or its workers had provided any help in the renovation. He said he believed that the work had been done by other contractors whom his wife paid.