WASHINGTON -- Virginia Gov. Tim Kaine has told close associates that he has had "very serious" conversations with Sen. Barack Obama about joining the Democratic presidential ticket and has provided documents to the campaign as it combs through his background, according to several sources close to Kaine.

Sens. Evan Bayh of Indiana and Joe Biden of Delaware are also being seriously vetted by the campaign staff, according to sources with knowledge of the process.

Obama has revealed little about which way he is leaning. And despite rising anticipation that a decision is imminent, campaign officials said an announcement is likely in mid-August, shortly before the Democratic National Convention. Obama's top aides, David Plouffe and David Axelrod, huddled Monday in the Washington office of Eric Holder, who along with Caroline Kennedy is vetting potential vice presidential picks.

Although rumors have circulated about former military leaders and other nontraditional contenders, including Republicans, Obama's circle of prospects is heavy on veteran senators with foreign policy experience. Kaine and Gov. Kathleen Sebelius of Kansas are the only state leaders believed to be under serious consideration, according to sources close to Obama.

Aides to Kaine declined to comment about the possibility that Obama might pick him, referring all questions to the senator's campaign.

"The governor has been pretty clear from the beginning, when Senator Obama asked him to be a national co-chair, that any conversation he has with the campaign, on any topic, are conversations that he is keeping private," said Delacey Skinner, Kaine's communications director.

But several people who have spoken to Kaine directly said he has talked about the seriousness of the possibility. All requested anonymity, citing the campaign's desire to keep the process secret. One said Kaine has stressed that there are other top candidates but described his discussions with Obama's campaign as "very serious."

Two other associates said Kaine's staff is providing the background research necessary to allow Obama's campaign to search for potential political land mines.

Since campaigning together during Kaine's 2005 gubernatorial race, he and Obama have become friends. Kaine has campaigned across the country for the Illinois senator during the primaries. In recent weeks, Kaine and his staff have been in frequent contact with Obama and his campaign about strategy and operations in Virginia. Kaine has said he plans to attend the Democratic convention in Denver.

Picking Kaine would seem to satisfy many considerations Obama has recently laid out. During an interview with Tom Brokaw on NBC's "Meet the Press" on Sunday, the Democratic nominee said he would look for someone who shares his desire to change Washington politics.

Kaine, a governor and former Richmond mayor, would bring outside-the-beltway credentials to the campaign. The relationship the two share would seem to fit with Obama's desire for someone "with independence -- who's willing to tell me where he thinks, or she thinks, I'm wrong." And Kaine would likely bolster Obama in Virginia, where the campaign is making an all-out push.

But Kaine has no foreign policy background, and as a first-term governor, he may add to voters' concerns about Obama's lack of experience. Kaine remains popular in Virginia, but he has had trouble dealing with Republicans and has no single defining achievement to point to on the campaign trail.

In interviews, Obama has hinted that experience would factor into his thinking as he decides on a running mate.

"I want somebody who I'm compatible with, who I can work with, who has a shared vision, who certainly complements me, in the sense that they provide a knowledge base or an area of expertise that can be useful. Because we're going to have a lot of problems and a lot of work to do," he said Sunday. "I want somebody who's going to be able to roll up their sleeves and really do some work."

Speaking on Fox News Sunday, the Weekly Standard's William Kristol said he believed Obama has already settled on Kaine.

Obama is "in Washington on Tuesday. ... He'll have a secret meeting with Tim Kaine -- this is my theory -- they'll work it all out," Kristol said. "And then on Monday, next Monday, August 4th, 11 a.m. in Richmond, Obama and Tim Kaine, and that will be an attractive young ticket. ... I'm way out there on a limb here."