Arizona Sen. John McCain will take the first step Thursday in what could be a long journey toward the Republican nomination for president in 2008, when he files paperwork with the Federal Election Commission.

McCain is expected to have plenty of company, as Tommy Thompson, former Health and Human Services Secretary and former governor of Wisconsin, also said he'll throw his hat in the ring, too. Both will join former New York Mayor Rudy Guiliani, who launched his exploratory bid earlier in the week.

McCain launched a Web site — www.exploremccain.com — that allows supporters to donate and join his effort. His speech before the conservative group GOPAC on Thursday will be broadcast on the site.

The move was expected — the senator discussed it Sunday on national television. McCain insists he won't make a final decision about running for president until after the Christmas holidays.

McCain, a former Navy pilot who was a prisoner of war in Vietnam, ran for president against George W. Bush in 2000 and lost in a bitter two-way race.

If McCain were to run again, he would turn 72 on Aug. 29, 2008, at the height of the campaign. Only President Reagan was older — 73 at the start of his second term.

Meanwhile, Thompson also jumped into the race as a possible presidential contender, announcing his plans after meeting with about 100 members of a group called Iowans for Wellness and Prevention.

Thompson told the crowd that he intended to file papers "after the first of the year."

The Iowa Caucases are the first formal beauty contest in the presidential nominating process.

Thompson, who will turn 65 on Sunday, spent nearly four decades in politics and government, including 16 years as governor. He resigned as HHS secretary in December 2004 shortly after Bush won a second term. His tenure at HHS was marked by anthrax attacks and a flu vaccine shortage.

Thompson was also a key player in Bush's AIDS initiative, a commitment of $15 billion (euro11.7 billion)over 5 years for treatment and prevention of the disease overseas. Thompson traveled frequently to Africa.

Thompson had used his AIDS role to trumpet his idea of medical diplomacy, investing in health care and medical facilities around the world.

Under federal election law, an exploratory committee allows an individual to travel and gauge the level of support for a candidacy without formally declaring themselves a candidate and adhering to all the federal rules of fundraising. An individual who spends money only to test the waters — but not to campaign for office — does not have to register as a candidate under the election law.

Former New York City Mayor Rudy Giuliani, also a Republican, confirmed this week that he has filed papers to start an exploratory committee.

The Republican field is expected to grow with Massachusetts Governor Mitt Romney, Sen. Bill Frist of Tennessee, Arkansas Gov. Mike Huckabee and New York Gov. George Pataki likely to join the presidential fray.