John Ensign, the Nevada Senator still reeling from an adultery and corruption scandal, will resign tomorrow, according to several published reports. Rep. Dean Heller, who has already announced a run for Ensign’s seat in the Senate, will be appointed as an interim replacement for Ensign by Governor Brian Sandoval, giving him the power of incumbency and a leg up in what should be a competitive Senate election in 2012. This leaves Heller’s seat open, and a special election will ensue.

Ensign already announced back in March that he wouldn’t run for re-election when his term expired in 2012, but this resignation was not expected. Venerable Nevada journalist Jon Ralston broke the news, and he said that a pending ethics investigation was the reason for the resignation. Ensign has already beat several investigations of his adulterous affair with the wife of a top aide, and his subsequent payoff to Doug Hampton and his wife, including him scheming to get Hampton a lobbying job. But the Senate Ethics Committee investigation has survived and continued, and Ensign may have felt things were getting a bit too dicey there.

In addition, there’s a political benefit for Republicans. Heller’s appointment puts him in the incumbent position over 18 months before the election. Not only would this help him in a hypothetical general election against Democratic Rep. Shelley Berkley, but it will help him enormously in an expected Republican primary. Heller is considered the candidate with the best chance of holding the seat for the Republicans in 2012.

As for NV-02, the seat given up by Heller, there will have to be a special election, though the process and the timeframe is unclear. The Governor must call for the election within a week of the vacancy, but he could set the date up to 180 days from then. It appears that the state parties nominate a candidate for the special election internally. This could be a winnable pickup for Democrats, though Heller survived both wave elections in 2006 and 2008, and the seat leans Republican.

UPDATE: Ensign’s full statement:

It is with tremendous sadness that I officially hand over the Senate seat that I have held for eleven years. The turbulence of these last few years is greatly surpassed by the incredible privilege that I feel to have been entrusted to serve the people of Nevada. I can honestly say that being a United States Senator has been the honor of my life. I know that my staff has been devoted to helping those in our state when they needed our assistance the most, and I hope that this will be the enduring legacy that we leave behind. As I close the door to this chapter of my life, I am left with memories that I will forever treasure. Traveling across our state during the years, I have heard incredible stories of strength, of struggle, and of compassion. I have met people who have forever changed me, and I can say that I will go through life a better person because of the struggle that I have been through and the support that I have received. While I stand behind my firm belief that I have not violated any law, rule, or standard of conduct of the Senate, and I have fought to prove this publicly, I will not continue to subject my family, my constituents, or the Senate to any further rounds of investigation, depositions, drawn out proceedings, or especially public hearings. For my family and me, this continued personal cost is simply too great. I am gratified that, after extended investigations, both the Department of Justice and the Federal Election Commission saw no grounds on which to charge me with improper conduct. I was hopeful that, with the closure of these investigations against me the wear and tear on my family and me would soon be over. This was not the case. As is its right, the Senate Ethics Committee is continuing its investigation of issues into which it has been inquiring for the past year and a half. Indeed, the Committee even decided recently to devote more resources to its investigation by hiring an outside counsel even though the issues have been viewed and reviewed by so many others. I came to office with the pledge to make this country a better place, but that pledge does not end with my resignation. I will continue to fight for a better country and for a future that our children deserve. I cannot fully express what serving the people of Nevada has meant to me, but I will try to repay them for their trust for the rest of my life. To the people of Nevada, I humbly say thank you for what you have given to me through the years. To my family, thank you for the support and love that you have shown me. To my staff, thank you for coming on this incredible journey with me and for standing by me despite the obstacles.