Seeking to fend off a fade in the polls, Fred Thompson is proposing a wide array of tax cuts while stepping up attacks on two of his top rivals, Mayor Giuliani and Michael Huckabee.

The former Tennessee senator unveiled his tax plan in a Fox News appearance yesterday, calling for a slash in the rates for corporations and capital gains, a repeal of the alternative minimum tax, and a permanent extension of President Bush’s tax cuts.

The most striking element of his platform, leading fiscal conservatives said, was his endorsement of a proposal to create a separate, optional flat tax system in which individuals earning up to $50,000 a year would be taxed at a rate of 10% while wealthier Americans would pay 25% of their income. The “opt-in” flat tax was recently offered as a proposal by the conservative Republican Study Committee in the House, and Mr. Thompson is the first presidential candidate to announce his support for the idea.

He rolled out his tax plan on a day of partisan bickering in the Republican presidential race, as the candidates battle for support among conservative voters in the run-up to the Iowa caucuses and the New Hampshire primary in early January.

Mr. Thompson’s principal targets were Mr. Giuliani, who he said “has not … five minutes of experience” on national security issues, and Mr. Huckabee, the former Arkansas governor who has risen to second place in the Iowa polls. They all trail Mitt Romney, who has led the contest for months.

Citing Mr. Huckabee’s support for tax increases in Arkansas, Mr. Thompson called him a “very high tax governor” and said he “did everything he could as governor to keep the state Legislature from restricting illegal immigration.” He also opened a new line of criticism on Mr. Huckabee’s position on abortion, saying he had been inconsistent in his support for a constitutional amendment outlawing the practice nationwide. Mr. Thompson supports overturning the Roe v. Wade decision to allow the states to decide the question on their own, and he suggested that Mr. Huckabee, who has relied on an unimpeachable stance of opposing legalized abortions, was closer to that position than he has let on.

In a statement responding to the former “Law & Order” star, Mr. Huckabee’s campaign manager, Chip Saltsman, called his attacks “desperate” and said they were “evidence that Fred’s show is running out of material and he’s left to cranking out re-runs.”

The Huckabee campaign has relished the newfound attention from the former governor’s rivals, viewing it as evidence that he poses a genuine threat to pull off a win in Iowa. A spokeswoman to Mr. Thompson, Karen Hanretty, acknowledged the increased focus on Mr. Huckabee, saying he had been a “silly little side attraction” in the campaign until recently. “The media has given Huckabee a real free pass,” she said.

Mr. Thompson appeared to betray a similar frustration yesterday as he lashed out at coverage of his campaign by Fox News during his interview with Chris Wallace. After Mr. Wallace cited a negative “buzz” surrounding Mr. Thompson’s campaign and his recent slide in the polls, Mr. Thompson said that has been a “constant mantra” on Fox. “From Day 1, they said I got in too late. I couldn’t do it,” he said, before reciting praise from National Review magazine and his second-place standing in national polls and in South Carolina, an early primary state.

Mr. Thompson has held onto second place in many national surveys, but his support has dropped in recent weeks. He is running third or fourth in recent Iowa polls, and he has slipped to the single digits in New Hampshire.

His tax proposals drew a mix of praise and skepticism from influential advocates of fiscal conservatism. The president of the Club for Growth, Pat Toomey, said in a statement that Mr. Thompson’s support for an optional flat tax set him apart from other Republican hopefuls. He called the proposals “the kind of plan economic conservatives can rally around.”

The president of Americans for Tax Reform, Grover Norquist, said in an interview that Mr. Thompson’s backing of the optional flat tax was “extremely helpful” but that his plan was not achievable with Democrats holding majorities in Congress. “The bad news is it’s an imaginary wish list of things you would do if [Republicans] had control of the House and the Senate and the presidency,” Mr. Norquist said. He reiterated his call for Mr. Thompson to sign the group’s pledge to oppose any tax increases as president. “What’s missing is: What if you’re on defense?” he said.

Advisers to Mr. Thompson said he opposed all such pledges but pointed to his record of standing against tax increases throughout his career. “The best defense is a good offense,” one top adviser, speaking on the condition of anonymity, said in response to Mr. Norquist’s criticism. A president, he said, should “put forward a plan and not necessarily sit back and play defense.”