WASHINGTON — Sens. Lindsey Graham and Joe Lieberman, two advisers to John McCain's presidential campaign, have resigned their positions on a veterans' advocacy group that was posting Internet ads critical of Democrat Barack Obama.

Graham, a South Carolina Republican, and Lieberman, an Independent from Connecticut, stepped down as honorary members of the board of Vets for Freedom, citing a new McCain campaign policy on conflicts of interest.

Vets for Freedom has placed two ads on its Web site in which veterans prod Obama to visit Iraq and to meet with Gen. David Petraeus, the top U.S. commander in Iraq. The ads echo a message McCain has recently made on the stump.

Independent groups and political campaigns cannot coordinate their efforts. To that end, McCain earlier this month adopted a policy that states: "No person with a McCain campaign title or position may participate in a 527 or other independent entity that makes public communications that support or oppose any presidential candidate."

Graham and Lieberman often travel with McCain and went to Iraq with the Arizona senator in March. Both men have campaigned as surrogates for McCain.

A 527 group, named after the section of the tax code that created them, can raise unrestricted contributions, unlike political action committees.

Vets for Freedom is not a 527. It is a nonprofit 501(c)4 advocacy group that can also raise unrestricted money. But it also operates a political action committee, which is limited to individual contributions of no more than $5,000.

The Vets for Freedom Web ads were paid for by the group's PAC.