What issue could prove so important that senators would risk blowing up the vision of a GOP unified against spending? Earmarks, of course. You know, the ugly little spending perks that have grown into one of Washington’s biggest political liabilities. The pork that earlier this year was unilaterally sworn off (to public praise) by House Republicans, who appear to be ready to do the same even in their new majority.

In the Senate, not so much. South Carolina’s Jim DeMint is offering his party its first opportunity to prove it meant what it said, by offering up a moratorium on Republican senators’ earmarks. Fifteen GOP senators—including six senators-elect—are co-sponsoring the ban, which will get its vote on Tuesday. At least 13 Senate Republicans—spearheaded by Minority Leader Mitch McConnell and Sen. Jim Inhofe—are going to the mat to keep the spending privileges. Twenty senators are apparently still mulling over the complex decision of whether to demonstrate some principle.

And so, instead of the GOP leading the debate on spending freezes or ObamaCare, the newspapers and radio programs are filled with Mr. Inhofe declaring an “all out war” against those in his party trying to demonstrate some spending restraint. The Oklahoman, who is looking forward to the upcoming highway spending bill, went further to lament that so many Americans have become “brainwashed” on the earmark issue. That’s it. Blame the voters. It worked so well for Democrats.

Instead of Republicans using their air time to pressure President Obama over taxes, they are using precious hours to explain why earmarks are good, moral, even a constitutional duty. Americans are told, on the one hand, that earmarks are too small a budget item to merit this much attention yet, on the other hand, are too important for funding big, vital projects to relinquish. Untangle that one. Another favorite is that the earmark debate is siphoning attention away from more “serious” budget debates. Because Republicans can’t walk and chew gum at the same time?

Instead of the GOP giving its grass-roots something to cheer about, it has created its first fault line. The antagonized Tea Party Patriots, an umbrella group, has now sent out an email to 134,000 members urging them to do “battle” with a Republican Senate. The conservative blogs are already slamming the party.

Sen. Jim DeMint is offering Republicans a chance to prove they meant what they said on spending.

Yes, those defending earmarks have some arguments. Many Republicans are bitter over the degree to which the Obama administration has directed stimulus and budget dollars primarily to blue states; a unilateral GOP earmark ban will make it harder to combat that practice. Some are frustrated by the moratorium’s broad definition of an earmark, which might make it harder for Republicans to set spending priorities at the committee level—personal earmarks aside. Fair enough.

And today, irrelevant. The time for those arguments was long ago—before Congress had so abused the earmarking practice as to earn the nation’s ire. Republicans trying to convince voters they just don’t understand the merits of some earmarks sound like President Obama telling voters they don’t understand the wonders of ObamaCare. Voters are in no mood to be patronized; they want earmarks to end.

That goes too for GOP complaints that the moratorium isn’t “fair”: It’s nonbinding (which means any Republican can cheat) and doesn’t apply to Democrats. Independent voters didn’t pull the lever for Democrats—that, hello, was the point of the election. They voted for Republicans on the promise that, this time, the party would demonstrate spending discipline.

All those Republicans complaining that earmarks are a tiny part of the budget, and this just a “symbolic” vote? They’ve got one bit right. This is a hugely symbolic vote. The public is watching. And it is long-term credibility at stake. A GOP that is indeed intent on “serious” budget cuts is going to need public support for tough cuts. As GOP House Whip Eric Cantor has pointed out, a public that can’t trust Republicans to fix “small” problems won’t trust them on “large” ones.

And as one Senate aide told me this week—for a caucus that has been working so hard to set expectations, this would be a huge missed opportunity. Republicans are not going to be able to repeal ObamaCare, are unlikely to be able to permanently extend the Bush tax rates, and will struggle to roll back the worst Obama regulations. But an earmark ban is one expectation they can fulfill. Immediately.

Of the Senate Republicans who have yet to make up their minds—Jon Kyl (Ariz.), John Thune (S.D.), Mike Johanns (Neb.), James Risch (Idaho)—many have solid voting records and have supported bans in the past. They’re in the uncomfortable position of having to vote against their leader. Not fun. Then again, the alternative is to vote against the public—which has made its own position crystal clear.

Write to kim@wsj.com.