If a single-issue candidate can’t gain traction when his signature issue catches fire, why does he keep running?

That’s the question raised by Congressman Tom Tancredo’s longshot bid for the Republican presidential nomination. The GOP base is hungry for a leader who will take a hard line against illegal immigration. Instead President Bush and John McCain have teamed up with Ted Kennedy in support of a bill that offers conditional amnesty to millions of illegal aliens.

This development is thought to be a major cause of the recent 40 percent drop in small-donor contributions to the Republican National Committee. The Senate immigration bill has been denounced by conservative bloggers, columnists, radio talk show hosts, and activists. A Rasmussen poll found that only 26 percent of Americans favor its enactment.

Yet Tancredo, the man immigration restrictionists consider America’s most valuable politician, remains stuck in the 1 percent range in national polls. Any boost from the current amnesty uproar to his presidential campaign is invisible to the naked eye.

More puzzling, the latest immigration debates don’t even seem to have raised Tancredo’s profile.

By contrast, Ron Paul has been much more successful at using his presidential candidacy to gain a wider hearing for his opposition to the Iraq war — a much less popular position among the Republican faithful than support for a border security fence — and general libertarianism, even if he hasn’t yet gotten much of a bounce in the polls. Former Arkansas Gov. Mike Huckabee has at least earned high marks for his homespun debate performances.

Meanwhile, Tancredo is struggling to distinguish himself from Duncan Hunter, Tommy Thompson, and Jim Gilmore.

Tancredo’s performance in last night’s Republican debate at Saint Anselm College in New Hampshire showed why. Given an easy question on his main issue — and an opportunity to pound the unpopular Senate bill — he botched it by giving a kitchen-sink answer that maladroitly summarized his case against current immigration policy.

More people will remember his story about Karl Rove — complete with a vow to keep George W. Bush from darkening the White House’s door — than will recall anything he said about the Senate immigration bill.

His rivals didn’t make the same mistake. Rudy Giuliani, a past supporter of amnesty who presided over a sanctuary city in New York, skillfully slammed the immigration bill’s lack of unifying purpose. Mitt Romney, another former McCain-Kennedy backer whose current position on what to do with the existing illegal immigrant population is basically incoherent, also scored points when he said the federal government should enforce the laws already on the books. Indeed, Romney has rather improbably established himself as the field’s leading critic of the Senate bill, much to McCain’s obvious consternation.

It’s Tancredo who has the longest record on the issue, however. His Congressional Immigration Reform Caucus has grown to include a majority of the House Republican Conference. The contingent played a key role in killing last year’s Senate immigration bill — and may help defeat the present one as well. He champions the attrition through enforcement strategy that is the leading alternative to a path to citizenship for illegal immigrants.

So why are the Johnny-Come-Lately candidates stealing his thunder? They are smoother. They are quicker with effective sound bites. And they are more serious about running a winning presidential campaign.

Perhaps Tancredo senses that things aren’t going well. He seems to be shifting his focus from winning the GOP nomination to defeating the Senate immigration bill, which he calls the “Kennedy-McCain-Bush sellout of America.” But to move beyond preaching to the converted, his presentation needs some work — think less Archie Bunker and more Barbara Jordan.

If he can’t do it, the candidate might ask himself whether he is actually helping his cause.