If Steven's has an ounce of intelligence in him he'll drop out in September so the GOP can appoint a legit candidate for the nomination (Parnell, Lesil Mcguire, etc.)
That wouldn't be a very intelligent move on Stevens' part at all. This way, he can obstruct his trial while he runs for reelection; if he were to stand aside, the trial would proceed more quickly. Don't expect most, if any, politicians to have the party at heart.
Also, I missed this on first look, but Stevens has a 50% favorability rating! (Begich's is 63%.) That's easily enough to carry Stevens through a primary.