Question: The 2000 United States Senate election winner later resigned amid what investigation?

Context paragraphs:
United States Senate career of Hillary Clinton: Hillary Rodham Clinton served as a United States Senator from New York from January 3, 2001 to January 21, 2009.  She won the United States Senate election in New York, 2000 and the United States Senate election in New York, 2006.  Clinton resigned from the Senate on January 21, 2009 to become United States Secretary of State for the Obama Administration.

United States Senate election in Delaware, 2000: The 2000 United States Senate election in Delaware was held on November 7, 2000, in conjunction with the 2000 U.S. presidential election, other elections to the United States Senate in other states, as well as elections to the United States House of Representatives, and various state and local elections.  Incumbent Republican U.S. Senator William Roth ran for re-election to a sixth term, but was defeated by Governor Tom Carper.

United States Senate election in Indiana, 2010: The 2010 United States Senate election in Indiana took place on November 2, 2010, alongside 33 other elections to the United States Senate in other states and elections to the United States House of Representatives and various state and local elections to fill Indiana's class III United States Senate seat.  Incumbent Democratic U.S. Senator Evan Bayh decided in February 2010 to retire instead of seeking a third term shortly after former U.S. Senator Dan Coats announced his candidacy for Bayh's contested seat.  No Democratic candidate submitted enough signatures by the deadline to run, leading Democratic officials to choose U.S. Congressman Brad Ellsworth to be the nominee.  The Libertarian Party nominated YMCA instructor Rebecca Sink-Burris, who had previously run against Evan Bayh in the United States Senate election in Indiana, 1998 but with less success than in this election.  Republican nominee and former U.S. Senator Dan Coats won the open seat.

United States Senate election in Tennessee, 2006: The 2006 United States Senate election in Tennessee was held on November 7, 2006.  The election winner, Bob Corker, served his term between January 3, 2007 and January 3, 2013.  Corker replaced Republican Bill Frist in the Senate, who retired upon the end of his second term in 2007.  Corker was the Republican nominee, and the Democratic nominee was Harold Ford, Jr..  The race between Ford and Corker was one of the most competitive Senate races of 2006, with Corker winning the race by less than three percent of the vote.  Corker was the only non-incumbent Republican to win a U.S. Senate seat in 2006.  Since 1994, the Republican Party has held both of Tennessee's Senate seats.

United States Senate election in Nevada, 2000: The 2000 United States Senate election in Nevada was held on November 7, 2000.  Incumbent Democrat Richard Bryan decided to retire, instead of seeking a third term.  Republican nominee John Ensign won the open seat.

Plebgate: The "Plebgate" (or "Plodgate", "Gategate") scandal in the United Kingdom concerns an altercation between Conservative MP Andrew Mitchell, the Government Chief Whip at the time (who later resigned because of the incident), and the police, which took place on 19 September 2012.  It gained notoriety initially for the conduct claimed of Mitchell and again two months later when, subsequent to Mitchell's resignation, CCTV and other evidence was revealed which appeared to call into question some of the evidence against Mitchell.  Leaked police logs, later apparently backed up by eyewitness evidence, suggested that Mitchell had sworn at police officers on duty at Downing Street and called them "plebs" (a pejorative word signifying someone of low social class) when they refused to open the main gate for him as he attempted to leave with his bicycle.  Mitchell apologised but denied using the words claimed and in particular calling police officers "plebs".  However, finding his position untenable amid the media storm surrounding the incident, he resigned from office a month later.

United States Senate election in Vermont, 2000: The 2000 United States Senate election in Vermont took place on November 7, 2000.  Incumbent Republican U.S. Senator Jim Jeffords won re-election to a third term in office.  Jeffords switched from being a Republican to an independent, who would caucus with the Democratic Party, thus breaking the 50-50 lock.  As of 2017, this is the last Senate election in Vermont won by a Republican.

John Ensign: John Eric Ensign (born March 25, 1958) is an American veterinarian and former politician based in Las Vegas, Nevada.  He was a Congressman and United States Senator from Nevada, serving in the latter seat from January 2001 until May 2011.  He resigned amid a Senate Ethics Committee investigation.

United States Senate career of Barack Obama: The United States Senate career of Barack Obama began on January 3, 2005 and ended on November 16, 2008.  He resigned his seat in the U.S. Senate upon being elected President of the United States.  Obama won the seat in an election against Alan Keyes who replaced Republican Primary election winner Jack Ryan.

United States Senate election in Missouri, 2000: The 2000 United States Senate election in Missouri was held on November 7, 2000, to select the next U.S. Senator from Missouri.  Incumbent Senator John Ashcroft lost the election to Mel Carnahan, despite the fact that Carnahan had died in a plane crash three weeks before election day.

Supporting paragraphs: United States Senate election in Nevada, 2000 John Ensign
Answer: Senate Ethics Committee investigation