Speculation over whether Fred Thompson is serious about running for president just went toes-up. Mr. Thompson’s not-yet-a campaign has confirmed: He’s dipping his toes in.

Specifically, a Thompson adviser told The New York Sun yesterday, he will announce the formation of a presidential “testing-the-waters” committee early next week — possibly as early as Sunday.

A “testing the waters” committee is a step before the more familiar presidential exploratory committee. It allows the former Tennessee senator to raise money and hire staff. But it also prevents him from doing a number of other things: advertising his candidacy, referring to himself as a real candidate (presumably just in public, he can say whatever he likes in front of the bathroom mirror), raising money that could be transferred to another candidate, or raising money to get on the ballot.

The Thompson adviser I spoke to yesterday wanted to downplay the significance of the forthcoming announcement (“Don’t expect huge fanfare,” were the adviser’s exact words). But for the Republican Party, this is clearly huge.

For months, the Republican primary field has been dominated by candidates that many conservatives believe are deeply flawed. The front-runner, Mayor Giuliani, is widely believed to be too socially liberal to win the nomination. Lagger Senator McCain is a least favorite of conservatives everywhere and has done himself no favors in recent weeks by being one half of the hated Kennedy-McCain immigration compromise. Mitt Romney says a lot of the right things to social conservatives, and he says them slickly — but therein lies his problem, as he pulled the opposite routine in liberal Massachusetts for years.

Into this field has lumbered the 6-foot-6-inch elephant in the room: a homespun actor-politician with a Southern drawl and a reliably conservative voting record. (Or, at least relatively reliable … We’ll see what his opponents are able to do with it, now that he’s getting his feet wet.)

Ever since Mr. Thompson left the door open to a possible candidacy in a Fox News interview on March 11, his shadow has been looming larger and larger over the campaign. Almost immediately, Mr. Thompson overtook Mr. Romney in national and some state polls. More recently, Mr. Thompson has been nipping at Mr. McCain’s heels. And much of his support seems to be coming from Mr. Giuliani’s hide, as the former mayor’s lead narrows over the rest of the pack.

And all of this without Mr. Thompson’s having made a single serious move toward entering the race.

Well, now that move has arrived, and we’ll get to start seeing how it will shake up the field.

“A lot of conservatives have been struggling here to figure out where to go with a candidate that can keep the coalition together,” a former presidential candidate who is now president of the nonprofit American Values, Gary Bauer, told me yesterday evening, referring to the GOP alliance among economic, defense, and social conservatives. “I think Fred Thompson has a fairly decent chance of emerging as the candidate that can do that.”

I had to ask him to clarify whether this was an endorsement. He said it wasn’t.

I then asked whether Mr. Romney might also fit the bill.

“Mitt Romney is trying to fill that space,” Mr. Bauer said. “But he continues to wrestle with concerns about how recently and how sincerely he has come to those [socially conservative] positions.”

“Meanwhile,” he said, “Thompson has a record that goes back quite a few years.”

“It will make the competition better,” he concluded.

That it certainly will. Those toes could end up making big waves.

rsager@nysun.com