So far as a head-to-head match-up is concerned, Sen. John McCain doesn't have much to fear from Rep. Tom Tancredo, one of the GOP's "issue candidates" whose issue is to oppose the very comprehensive immigration reform McCain supports. But McCain does have something to fear from Mitt Romney, who has capitalized on Tancredo's role in the race by criticizing McCain for the so-called "McCain-Kennedy" immigration reform bill from last year.

With that in mind, here's the lead of a Boston Globe story today, "Kennedy-McCain Partnership Falters":

WASHINGTON -- Senators Edward M. Kennedy and John McCain have all but abandoned plans to cosponsor a comprehensive immigration reform bill this year, as McCain faces tough questions from conservatives on the presidential campaign trail about his support for immigrants' rights.

Not more than a week ago, hopes were high that Congress would be able to pass a comprehensive immigration bill that President Bush could sign. The Globe's story says the outlook now isn't so good.

The erosion of the unlikely political partnership that brought the liberal Kennedy and the conservative McCain together on immigration suggests a tough road ahead for passing a sweeping immigration measure this year. Further complicating efforts to find consensus, a group of Republicans is working with the White House to draft an alternative bill.

The Globe story goes on to say that McCain isn't abandoning his support of comprehensive reform. But McCain certainly faces a problem with many conservative voters that he can't solve completely no matter what he does. A "flip-flop" on immigration would open McCain up to criticism of pure political opportunism. At the same time, he can't go into the primaries associated with Ted Kennedy, as in the "McCain-Kennedy" immigration reform bill. So it looks like McCain will take Option 3: Hang back.

This predicament isn't entirely fair to McCain, since almost all of the top GOP candidates favor the comprehensive approach also. Even Romney once called the McCain position "reasonable." But McCain is already associated with one too many bills the conservative base hates. Adding another would look like a deliberate provocation.

Now, why McCain takes all the heat for his more liberal positions, while Rudy Giuliani is seemingly (so far at least) given a free pass is another matter.

Update: Check out this March 16 Boston Globe article about Romney's recent criticism of McCain-Kennedy and how it contrasts with what he said as early as 2005. The Globe has included audio from his meeting with its reporters in 2005 as well as audio from Romney's CPAC appearance.

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