As Iowa Democrats go to the caucuses tonight to begin selecting a candidate to oppose President Bush in November, the most recent polls show a remarkable surge there in the candidacies of Senators Edwards and Kerry. All sorts of reasons have been offered for these surges, ranging from Mr. Edwards’s supposed abstention from negative campaigning to the passage of time since when the two senators voted in favor of the Iraq war. Our columnist Mark Steyn makes the point in the adjacent column that the collapse of Howard Dean’s support came remarkably quickly after the former governor of Vermont was endorsed by Vice President Gore.

Our own sense is that most of the Edwards and Kerry surge and the faltering of Dr. Dean is related to the issue of taxes.

Mr. Kerry hasn’t made a secret of this. “Howard Dean has a program to raise people’s taxes,” Senator Kerry said in a January 6 debate in Iowa. “He’s going to increase the burden on middle-class Americans.” The New Republic’s Ryan Lizza recently reproduced on the magazine’s Web site a copy of a mailing that the Kerry campaign sent Iowa voters. One side of the mailing carried photos of Howard Dean and Richard Gephardt with the blaring red warning: “These two candidates want hard-working middle class Iowa families to pay higher taxes.” The other side said, “Howard Dean and Dick Gephardt plan to RAISE TAXES on the middle class by thousands of dollars” — while pledging that Mr. Kerry would fight against middle class tax increases.

Senator Lieberman piled on, too, in that January 6 debate, criticizing Dr. Dean by saying: “I don’t know of a case where a Democratic candidate has been elected who called for a massive increase in taxes on the middle class.”

Senator Edwards has been sounding a theme similar to Mr. Kerry’s, and, as voters started in the past few weeks to focus on the race, he’s been gaining traction. “I would say to Governor Dean and Dick Gephardt, I grew up in a middle-class family whose taxes they’re talking about raising. For a family of four, who makes about $40,000 a year, we’re talking about almost $2,000; $2,000 that could be used to pay a lot of bills,” Mr. Edwards said in a September 25 debate in New York. “What we ought to be doing instead is empowering those families, helping them buy a house, helping them invest, lowering their capital gains rate. So we improve the — and expand the investor class in America.”

We don’t want to pre-judge the outcome of Monday’s caucus: Dr. Dean could yet pull it out. If he does, he’ll still have to face the New Hampshire primary. There, two of his opponents, Mr. Lieberman and General Wesley Clark, have gone beyond Mr. Kerry’s opposition to Dr. Dean’s proposed tax increases; they’ve actually proposed middle-class tax cuts of their own. “All taxpaying families with children making under $100,000 will get a tax cut,” General Clark vowed in a January 5 speech in Nashua, N.H. “I’m going to provide tax cuts to ease the burden for 34 million American families.”

In the general election, any of these Democrats will lack tax-cutting credibility against President Bush. Mr. Bush’s own already-implemented tax cuts are more growth-oriented than any of the Democrats’ promises because they cut the marginal tax rates of the top earners.

Until the general election, though, we find it an encouraging sight to see the Democrats on the national scene scrapping to be seen as tax-cutters and attracting a following within their own party on the issue. Partisan Republicans may rue the Democrats improving their standing on the tax issue. But if the framework of the debate changes from the old one — whether to cut taxes or raise them — to a new one — what’s the best way to cut them — the taxpayers are likely to emerge as the winners.