A timeline of major events leading to the announcement Thursday by Republican U.S. Sen. John Ensign of Nevada that he will resign May 3:
November 1998 _ U.S. Rep. John Ensign, a veterinarian and former casino executive, wins seat in U.S. Senate.
November 2006 _ Ensign defeats Democrat Jack Carter, former President Jimmy Carter's son; re-elected for second term in U.S. Senate. Doug Hampton, a former utility executive and close friend of Ensign's, hired as a top aide in Senate office. Hampton paid nearly $14,000 a month.
December 2006 _ Cindy Hampton, Doug Hampton's wife, hired to work for Ensign for Senate and Battle Born PAC campaign committees.
December 2007 _ Ensign and Cindy Hampton begin affair, according to accounts provided by Ensign's office and Doug Hampton. Doug Hampton discovers affair through text message and confronts Ensign on Christmas Eve, according to Hampton. "John cries like a kid. Puts his head in his hands, cries like a little boy," Hampton later told ABC.
February 2008 _ Cindy Hampton's pay at both political entities doubles to nearly $4,000 a month because she took on additional duties, according to Ensign's office. Ensign confronted about affair by U.S. Sen. Tom Coburn, R-Okla., and others at Washington residence.
April 2008 _ Ensign's parents, Mike and Sharon Ensign, give Hampton family $96,000 out of concern for the family's well-being, according to John Ensign's attorney.
May 2008 _ Doug and Cindy Hampton leave payroll. Hampton receives $20,000 for final month of employment, including nearly $6,000 for 12 days unused vacation, according to Ensign's office. Doug Hampton begins consulting work for November Inc., a firm founded by Ensign's closest adviser and run by former Ensign aides. Hampton lands contract with Las Vegas-based Allegiant Air, airline run by Ensign political contributor Maurice Gallagher.
August 2008 _ Affair between Cindy Hampton and Ensign ends, according to Ensign. Allegiant Air hires Doug Hampton as vice president of government affairs.
May-June 2009 _ Doug Hampton's attorney seeks money from Ensign, according to Ensign's office.
June 1, 2009 _ Ensign visits Le Mars, Iowa, fueling speculation of presidential ambitions.
June 11, 2009 _ Doug Hampton writes letter to Fox News Channel host Megyn Kelly asking Kelly to expose Ensign affair.
June 15, 2009 _ Fox News booker receives letter from Doug Hampton via email. Fox producers speak to Hampton and find him "evasive and not credible" and do not pursue story, according to a Fox News producer.
June 16, 2009 _ Knowing Hampton has contacted the media, Ensign releases statement acknowledging affair with ex-aide. Calls news conference to apologize.
June 17, 2009 _ Through a statement by a lawyer, Doug and Cindy Hampton acknowledge Cindy Hampton's affair with Ensign.
June 18, 2009 _ Ensign phones in resignation as member of Senate GOP leadership.
June 19, 2009 _ Ensign spokesman Tory Mazzola says Doug Hampton made "exorbitant demands for cash and other financial benefits" through an attorney.
June 23, 2009 _ Ensign apologizes to Senate colleagues over affair.
July 8, 2009 _ Doug Hampton says Ensign paid his wife severance, provides letter to Las Vegas Sun that he claims is a handwritten apology from Ensign to Cindy Hampton. "I used you for my own pleasure," the letter reads, later adding. "Plain and simple it was wrong; it was sin."
July 9, 2009 _ Ensign admits to $96,000 payment, saying his parents gave the money in a single check "out of concern for the well-being of longtime family friends during a difficult time."
July 14, 2009 _ Ensign says he has no plans to resign and intends to seek re-election in 2012.
August 2009 _ Ensign gets mixed reviews during two days of carefully scripted appearances in Nevada _ his first since announcing the affair. He promises a Republican women's club in Fernley that he'll make up for his "big mistake" by working harder than ever. He makes stops in later weeks in Henderson, Las Vegas, Elko, Alamo and Lovelock.
Aug. 19, 2009 _ Ensign tells The Associated Press: "I haven't done anything legally wrong."
October 2009 _ New York Times reports Hamptons sought $8.5 million settlement from Ensign before the affair became public.
Nov. 30, 2009 _ Ensign tells a radio talk show in Las Vegas that resigning would make it harder for Republicans to unseat Senate Democratic Majority Leader Harry Reid in 2010.
March 2010 _ A federal grand jury in Washington issues subpoenas to the National Republican Senatorial Committee, which Ensign had chaired, and to companies in Nevada in the Ensign probe.
May 2010 _ Senate Ethics Committee attorneys spend several days in Las Vegas questioning executives, Republican party strategists and Cynthia Hampton. Ensign files documents later that month with the Senate setting up a defense fund to pay legal bills.
August 2010 _ Ensign sends letters to supporters insisting he is "absolutely" innocent of allegations that led to Senate Ethics Committee and the Justice Department probes; seeks contributions to his defense fund.
October 2010 _ Financial disclosure reports show Ensign spent more than $550,000 in three months on legal fees for himself and aides drawn into the Senate Ethics Committee and U.S. Justice Department investigations.
Dec. 1, 2010 _ Ensign lawyers say the Justice Department is no longer targeting Ensign in a criminal investigation arising from his affair.
January 2011 _ Ensign sets a fundraising goal of at least $1 million by July for a 2012 re-election campaign. He later reports spending nearly $700,000 in legal fees in recent months, leaving him with barely $300,000 in campaign funds.
Feb. 1 _ The Senate ethics committee appoints a special counsel to investigate Ensign.
March 7 _ Ensign announces he won't seek re-election.
March 15 _ Republican U.S. Rep. Dean Heller ends months of hinting and announces he'll run for Ensign's seat.
March 24 _ Doug Hampton is indicted by a federal court grand jury in Washington, D.C., on charges he violated federal criminal conflict of interest laws.
April 14 _ Democratic U.S. Rep. Shelley Berkley announces she'll run for Ensign's seat.
Thursday _ Ensign announces his resignation. He denies violating laws or rules, but says he doesn't want to subject his family, friends and constituents to further investigations.