With new polls showing a dramatic tightening of his race against President Bush, Democratic presidential nominee John F. Kerry today lashed out at the administration over stem cell research, calling on Bush to lift "political barriers" that he said were imposed when Bush gave in to "extreme right-wing ideology" on the issue.

Bush today campaigned in the battleground state of Iowa, where he signed into law the fourth tax cut of his presidency. The bill, the "Working Families Tax Relief Act of 2004" that was passed by Congress last month, extends tax breaks for an estimated 94 million Americans.

The campaign appearances by Kerry and Bush in key swing states came as the latest opinion polls showed the two candidates in a statistical dead heat.

According to a CNN/USA Today/Gallup poll issued Sunday, Bush and Kerry are tied at 49 percent among likely voters, with independent candidate Ralph Nader far behind at 1 percent. The poll has a margin of error of plus or minus 4 percentage points. In the Gallup organization's previous poll, taken Sept. 24-26, Bush led Kerry among likely voters by 52 percent to 44 percent.

According to a Zogby International poll conducted Oct. 1-3 and released today, Bush leads Kerry by 46 percent to 45 percent in a two-way contest, with 8 percent undecided. This represents a narrowing of the race from a previous Zogby poll conducted Sept. 17-19 that showed Bush at 47 percent, Kerry at 44 percent and 7 percent undecided. The poll has a margin of error of plus or minus 3.1 percent.

When Nader is included, the latest Zogby poll shows Bush leading Kerry by 46 percent to 43 percent -- unchanged from the September poll -- with Nader at 2.4 percent.

Pollsters said the latest surveys appeared to reflect Kerry's performance in last week's debate with Bush, a contest that surveys immediately afterward said was clearly won by the Massachusetts senator.

"The close race got even closer," pollster John Zogby said in a statement that accompanied today's survey results. "There is some evidence that Kerry's debate performance and increased clarity on the war has helped him consolidate at least some of the support that he has lost. But undecideds are up to 8 percent, and there is still a month to go."

Zogby said there was "also no doubt that Ralph Nader is hurting Kerry." He said Bush's biggest electoral hurdle appears to be undecided voters, "who give him a 31 percent positive job performance rating and a 69 percent negative rating." He said his polling shows that only 13 percent of undecided voters feel that Bush deserves reelection -- his lowest percentage yet -- while 37 percent feel it is time for someone new.

In a town hall meeting in Hampton, N.H., with actor Michael. J. Fox, who suffers from Parkinson's disease, Kerry charged that Bush "has turned his back on science" through his decision three years ago to permit federal funding for a limited amount of research on embryonic stem cells and prohibit it for stem cell lines created after Aug. 9, 2001.

"The hard truth is that when it comes to stem cell research, this president is making the wrong choice to sacrifice science for extreme right-wing ideology," Kerry said.

Coinciding with the line of attack, the Kerry campaign released a new ad on the topic, which Democrats say is a popular one with their audiences.

In the 30-second ad, Kerry argues, "It's time to lift the political barriers blocking the stem cell research that could treat or cure diseases like Parkinson's. . . . There's no time to wait. At stake are millions of lives."

In reinforcing that message, Kerry told the town-hall style meeting in a Hampton, N.H., school gymnasium, "We stand at the next frontier, but instead of leading the way, we're stuck on the sidelines" on stem cell research. And Bush is "unwilling to change course," he said.

"The majority of the American people support stem cell research, and it's high time we had a president of the United States who does, too," Kerry said. "We can't afford any more stubborn refusal to face the facts."

Kerry listened to several tales from people hoping stem cell research could lead to cures for their afflictions. The invited crowd became emotional when a father, on stage with his diabetic son, held a large package of needles to show how many insulin injections the boy must have in a month.

Kerry was joined by Fox, the actor who has become a stem cell research activist since being stricken with Parkinson's. Fox told the gathering that Bush "has so restricted the stem cell lines available to us that it was kind of like he gave us a car and no gas and congratulated himself for giving us the car."

According to polling conducted for the Kerry campaign in July, 69 percent of voters support stem cell research, including majorities among Democrats, Republicans and independents.

Bush campaign spokesman Steve Schmidt said that Bush has not banned stem cell research and has, in fact, provided the first federal funding for such research.

"John Kerry's attacks on stem cell research are trying to mislead the American people by implying a ban that doesn't exist," he said.

Kerry has argued that the limitations imposed on embryonic stem cell research by Bush's August 2001 decision are so severe as to amount to a virtual ban.

According to biomedical researchers, the existing embryonic stem cell lines that were approved for federal funding three years ago are inadequate to achieve significant scientific advances. Although the Bush administration at the time said there were 64 such stem cell lines, the number that can actually be used is no more than 21, and all are contaminated with mouse cells, researchers say.

Appearing in Des Moines, Iowa, Bush focused today on his tax-relief policies and called on Congress to take further steps.

"This legislation will have a good effect throughout the economy," he said before signing the tax relief bill. He said that by extending key portions of earlier measures, "we will leave close to $50 billion next year in the hands of the people who earned it."

This, Bush, said, will result in more new jobs.

He said the legislation "is essential, but it's only a start," adding, "We need to make all the tax relief permanent."

Congress and the administration need to "make sure the death tax doesn't come back to life" and reform the tax code to make it simpler, Bush said. "We need spending discipline in our nation's capital," he added.

The latest tax relief package prevents three middle-class tax breaks from expiring Jan. 1 and renews a slew of others that benefit businesses.

The nearly $146 billion package maintains the per-child tax credit at $1,000 for five years, extends the provisions of a 10 percent income tax bracket for six years and retains relief from the measure known as the marriage penalty for four years. All told, an estimated 94 million Americans are expected to benefit from the individual taxpayer relief totaling $131.4 billion.

But the legislation also includes provisions added by congressional Republicans that extend 23 tax breaks for businesses that were due to expire.

Milbank reported from Hampton, N.H.