Roll Call, along with several other outlets, relayed the news before Ensign officially made his plans known.
"There are consequences to sin," Ensign said to members of the press, the Washington Post reports. He added that he did not want his family to have to deal with an "exceptionally ugly" campaign.
He also said at the time, "I have never felt like I broke any laws or ethics rules."
Ensign is currently the subject of a Senate Ethics Committee investigation related to the fallout from an affair he had with Cynthia Hampton, an ex-staffer of his campaign and wife of his best friend and co-chief of staff.
As early as April of last year, calls began to mount from Republicans for Ensign to resign over the controversy. The Las Vegas Sun more recently reported on the forces working against the incumbent lawmaker:
Still, Ensign lacks two things that would be necessary to wage a convincing fight. One is a war chest. The other is a party to back him up.
Nevada's GOP is in acute rebuilding mode after an ugly primary and 2010 midterm season in which several Republican leaders split from their base to back Democrat Sen. Harry Reid.
Influential GOP members say Ensign has never taken the time to build up the base, or fortify the favor-based friendships every politician needs to call in when he's accused of sleeping with his staffer's wife and covering it up with campaign funds.
Last week, Politico reported that the Senate Ethics Committee probe of Ensign was heating up.
As for who could run on the Republican side to replace Ensign in 2012, Rep. Dean Heller (R-Nev.) hinted that he could mount a campaign to challenge Ensign in a primary last month. Another possible GOP contender who could vie for the post is former Nevada candidate Sharron Angle, who was defeated by Senate Majority Leader Harry Reid in the 2010 midterm election season.
The Hill recently reported that Angle addressed the prospect she could run against Ensign in a primary match-up in 2012. "I've got many options and I'm not ruling out anything," she said on the possibility of mounting a primary campaign.
Sue Lowden, who Angle defeated in Nevada's 2010 Senate GOP primary, recently signaled that she would consider running for Senate next year if Ensign decided against seeking another term and depending on Heller's ambitions. "If John Ensign doesn't run, and Congressman Dean Heller decides not to throw his hat in the ring, I would think about it seriously," she said during an appearance on MSNBC.
Following the 2010 midterm election, the Las Vegas Review Journal reported on what's at stake for Republicans and Democrats as it relates to Ensign's seat in 2012:
It will be key for Republicans to hold as they try to take control of the Senate from Democrats in 2012 as they did the House in 2010. Only 10 GOP Senate seats will be up for grabs, including Ensign's, compared with 23 Democratic seats to defend.
With news of Ensign's intention to retire, only time will tell who will jump into the race to secure his seat. The Rothenberg Political Report kept its classification of the post as "lean Republican" following the senator's announcement on Monday.