Nevada senator John Ensign announced Monday he won't seek re-election in 2012. The Republican lawmaker has been embroiled in scandal since he admitted to an extramarital affair with Cynthia Hampton, the wife of one of his chief legislative staffers. Reflecting on the mounting pressures Ensign faced, inside and outside his party, it's somewhat remarkable this announcement didn't come sooner. Here's a look at the factors that likely hurried his decision:

He Faced a Brutal Primary, reports Michael Memoli and Lisa Mascaro at the Los Angeles Times: "The scandal threatened to overshadow Ensign's campaign and fundraising potential; Republicans have suggested he would not even have survived a primary election."

He Was About to Get Grilled by the Senate Ethics Committee  He's the subject of a "preliminary inquiry" into whether he broke any ethics rules during his affair with Hampton. Ensign allegedly paid Hampton and her husband hush money, explains USA Today's Catalina Camla:

Her husband, Doug Hampton, alleged the senator helped him secure lobbying work after he stopped working for Ensign as his deputy chief of staff. Ensign's parents also contributed $96,000 to Cynthia Hampton and her family.

His Party Has Been Begging For This The Washington Post's Chris Cillizza explains:

Senate Republican strategists will breathe a big sigh of relief that Ensign is now out of the race. Rep. Dean Heller, who had been preparing for a primary challenge to Ensign, will now almost certainly run and the party establishment will do everything in its power to clear the primary for him. It remains to be seen whether any of the GOP candidates who ran for Senate in 2010 -- including Republican nominee Sharron Angle -- will get into the race.

Lobbyists Have Been Pressuring Him to Leave, reported Roll Call's Kate Ackley in February:

At a recent NRSC fundraiser... several lobbyists urged the chairman to get Ensign out of the race, sources said. The lobbyists have been feeling the pressure from Ensign's fundraising operation to donate, and they suggested... that it wasn't in the party's best interest to give the Nevadan money.

"There is a problem with dangerous, vulnerable Members like John Ensign, who most people downtown don't believe has cleared his name," another Republican lobbyist said. "We're all asking, 'What are you doing?' And we're asking [the NRSC], 'What are you guys doing to deal with this?'"

His Legal Bills Kept Piling Up, reported Roll Call's Jennifer Yachnin in January:

Sen. John Ensign doled out more than $97,000 from his legal defense fund for attorneys in the final months of 2010, according to his most recent report to the Secretary of the Senate. 

The Nevada Republican has been the subject of multiple investigations into his alleged efforts to cover up an affair with the spouse of his former top aide.