This post has been updated

The day after news broke that a new super PAC will be aiding the Senate campaign of George Allen (R), Timothy M. Kaine (D) proposed — and Allen quickly rejected — the idea that the two foes strike an agreement to force outside groups spending money on their race to disclose their donors.

Former Virginia governor Tim Kaine, left, and former senator George Allen greet each other after the AP Day at the Capitol Senatorial debate in Richmond on Dec. 7, 2011. (Steve Helber/AP)

During their debate in December, Kaine suggested that the two candidates call on super PACs and outside groups to stay out of the race, an idea Allen dismissed.

“Since you are unwilling to exclude Super PACs entirely (as candidates in other states have done), how about basing this campaign on the Virginia principle you praised a few months ago?” Kaine asked in a letter to Allen on Tuesday, recalling that Allen had praised Virginia’s disclosure rules during their debate.

“Let’s adopt a simple rule: No Secret Money,” Kaine wrote. “Let’s commit that any group running ads or conducting electoral activity for either of us should have to disclose their donors.”

Kaine suggested that if Allen agrees to the basic idea, the two campaigns could work out a mechanism to enforce it.

In a response letter sent Tuesday evening, Allen dismissed Kaine’s idea, suggesting that “most will see it as an unfortunate gimmick, typical of the partisan gamesmanship playing out in Washington today.”

Allen also accused Kaine’s campaign of being “substantially funded by big-union bosses” — though without providing any specific evidence — and called on Kaine to endorse the idea of allowing workers in unionized shops to make voluntary rather than compulsory donations.

In January, Massachusetts Sen. Scott Brown (R) and his challenger, Elizabeth Warren (D), struck a novel pact — dubbed the People’s Pledge — to discourage outside groups from spending money on their race.

Since the candidates can’t legally force outside groups to stay out, Brown and Warren agreed that if either of them was the beneficiary of an ad campaign on television, radio or the Internet, their campaign would donate 50 percent of the cost of the campaign to the charity of the opposing candidate’s choice.

In Montana, Sen. Jon Tester (D) and Rep. Denny Rehberg (R) failed to reach a similar agreement after weeks of heated negotiations.