The race for the Republican presidential nomination has reached a potential turning point.

The irony is that the fates of the professional politicians who began their campaigns historically early last year may well depend on a reluctant combatant getting into the race about the time candidates would if this were a typical election cycle.

Fred Thompson’s impending entry into the race and downticks in Rudy Giuliani’s and John McCain’s poll numbers will make this a momentous summer, and one in which the focus of attention might switch from the Democratic contest between Sens. Hillary Clinton and Barack Obama, which has dominated the news media coverage until now.

There are a variety of potential Republican scenarios. Almost all of them depend on how Thompson fares in the coming months because presidential politics, after all, is the ultimate zero-sum game.

If Thompson’s support continues to grow, especially McCain and to a more limited degree former Massachusetts Gov. Mitt Romney, who trails in the national polls but leads in fund-raising and the early states of Iowa and New Hampshire, inevitably will see their presidential chances go down the tubes.

It is impressive that as a non-candidate, Thompson has so far elbowed his way past Romney in the national polls and pulled even with McCain. But, with his hat in the ring, Thompson will be judged by a different standard. We’ll see if he is just the fresh-face flavor of the month as the other candidates hope, or the southern fried version of Ronald Reagan that his supporters believe.

Front-runner Giuliani, too, could find himself in an uncomfortable situation should Thompson catch further fire. History tells us that the former New York mayor’s candidacy could be problematical should he face an opponent from the party’s southern base.

We’ll see if Thompson’s candidacy is as attractive in the flesh as on paper, but his expected entrance early next month will bring a geographic and cultural dimension to the race that has not been in the mix until now. It is one that could work to his benefit.

The Sun Belt has dominated the Republican Party for the last half-century. In recent decades, religious conservatives have carried great weight in the process that has consistently nominated right-of-center candidates who reflected those views and values.

Neither Giuliani – whose views on abortion, gay rights, and gun control are at odds with the GOP mainstream – nor Romney, whose position on some of those same issues has changed to adhere to the party consensus, fit that political profile of recent Republican nominees.

McCain’s Arizona residence and war-hero background would seemingly put him in greater stead with the GOP faithful. But, his maverick style has alienated many GOP voters who might otherwise see him as the heir to Reagan and the Bushes.

This void, and poll data showing a longing among the GOP faithful for additional candidates, led many party conservatives to recruit Thompson, a former assistant U.S. attorney and Watergate prosecutor turned actor, turned U.S. senator. He served eight years in Washington before returning to a Hollywood career.

His backers see him as a 21st-century Reagan – conservative on the issues and charming on television. But since he was late to get into this race, and has run only in Tennessee, there are serious questions about how well his zero-to-sixty candidacy will fare once it becomes real.

A Thompson-Giuliani primary fight would require the former New York mayor to reverse history, since it has been more than a half-century since the Republicans nominated a northeasterner for president. Ten of the last dozen Republican presidential nominees have been from the Sun Belt (if you count California) and the only two who weren’t – Gerald Ford and Bob Dole – both lost in November.

Thompson may turn out to be a bust. But Republican voters clearly are willing to give him an audition to be their leading man. He has come further faster than any non-candidate in recent history. He has the opportunity to shake up the Republican nomination process and potentially give the GOP a star.

It will be an interesting summer.