WASHINGTON (CNN) -- Former Sen. Fred Thompson will officially "test the waters" for a presidential run by beginning to raise money and hire staff as early as Friday, several sources close to Thompson tell CNN.

While the former Tennessee senator is not an official candidate for president, he is acknowledging interest in the race for the Republican presidential nomination and this action is viewed as a major step towards a possible White House run.

Thompson is not required to file paperwork with the Federal Election Commission at this stage, but would have to account for his actions -- such as raising money and hiring staff -- during this initial phase should he decide to become a full fledged candidate at a later date.

"He is entering the 'testing the waters' phase," said one of the sources close to Thompson. "It allows the senator to go out and gauge support."

Thompson, 64, held a conference call with more than 100 supporters Tuesday night and received financial commitments from "big Republican donors," the source said.

Thompson has been in contact with "a lot of Reagan and Bush 41 (George H.W.
Bush)-types who are supportive" of his candidacy, another source with knowledge of Thompson's thinking told CNN.

This source noted that Thompson will "likely" make appearances in some of the early primary and caucus states like Iowa and New Hampshire before an official announcement is made.

"That announcement could come as early as the first week in July," the source close to Thompson said. "However, that is if he gets in."

A CNN/Opinion Research Corp. poll conducted May 4 to 6 placed Thompson third among GOP candidates with 13 percent of the vote. Former New York City Mayor Rudy Giuliani had the most support with 25 percent, two points ahead of Sen. John McCain¹s 23 percent. Former Massachusetts Gov. Mitt Romney was fourth with 10 percent.

The same poll of registered Republican voters found only 20 percent were very satisfied with the field of GOP candidates and 26 percent were not satisfied. 50 percent said they were somewhat satisfied.

Thompson announced earlier this year that he was diagnosed with lymphoma in 2004, but one of his physicians told CNN that the disease is currently in remission.

Thompson was elected to the Senate in 1994 to fill the unexpired term of then-Vice President Al Gore and retired in 2003 to resume his acting career. He now lives in McLean, Virginia.

-- CNN's Candy Crowley, Mark Preston and John Helton