An all-but-declared candidate for months, Thompson will establish an official organization next week to weigh a White House bid while launching his first major fundraising effort. He also may visit early primary states in late June and could officially enter the race as early as the first week in July, according to the officials, who spoke on the condition of anonymity because the plans were not public.
Despite the movement, they cautioned that Thompson has made no final decision about whether to run or where and when to get in the race should he decide to go forward.
They said he is considering several options for a possible formal announcement, and that one scenario has him officially announcing his candidacy over the July 4 holiday. It's also possible, they said, that Thompson could wait until later in the summer to declare his intentions.
"Senator Thompson is still seriously considering getting into the presidential contest and he is doing everything he has to do to make that final decision," said Mark Corallo, a Thompson spokesman. "Stay tuned."
A summertime announcement has been widely expected and numerous signs have pointed to a Thompson candidacy, including his disclosure in April that he had been diagnosed with non-Hodgkin's lymphoma, a form of cancer, but is in remission. Officials say it's more likely than not he will join a crowded GOP field that's led by Rudy Giuliani, John McCain and Mitt Romney.
Thompson has been competitive with the top-tier trio in national popularity polls, which largely measure name recognition early in the campaign. He is well-known nationally for his acting role as gruff district attorney Arthur Branch on NBC's long-running drama "Law & Order."
His expected entrance in the race could dramatically shake up the GOP field, but it's unclear exactly who among the top GOP candidates — Giuliani, McCain or Romney — would be affected the most by his candidacy.
Thompson has been casting himself as a straight-talking conservative in the mold of former President Reagan — even though as a senator, he took some positions that angered the party's base.
According to a CBS News/New York Times taken last week, just 38 percent of Republican primary voters said they were satisfied with their current choices while 57 percent said they wanted more options in the race.
While former New York City Mayor Giuliani led in that poll, there are concerns among Republicans. The poll found that less than half of GOP respondents said that Giuliani shares Republican values. Many in the party are upset with McCain, once viewed as the party's front-runner, for co-sponsoring the immigration bill now under debate in the Senate. Romney, a former governor of Massachusetts, has surged in surveys of late but is still less well known than many others in the field.
"Conservatives are looking for a candidate they can buy into," said CBSNews.com Senior Political Editor Vaughn Ververs, "and for the moment at least, Fred Thompson is filling that void."