Republicans attacked Timothy M. Kaine on Thursday after he said during a much-anticipated Senate debate that he would consider a minimum income tax for every American, opening a fresh line of attack in a nationally watched race that until now has turned on mostly predictable and well-worn accusations.

Kaine, a Democrat, made the comment as he squared off against Republican George Allen, a fellow former governor and his opponent in the Virginia race, in their first televised debate. The hour-long program, hosted by “Meet the Press” host David Gregory at the Capital One Conference Center in McLean, was mostly devoid of fireworks.

But shortly after leaving the debate stage, Allen went after his Democratic opponent, telling reporters: “It’s typical of Tim Kaine. His record is always one of looking to raise taxes. When he was governor, he tried to raise taxes on people earning as little as $17,000 a year. He wanted to raise taxes on buying used cars.”

Kaine, who had said in the debate that he would be open to “some minimum tax level for everyone,” later qualified his statement, noting that he had not actually proposed a minimum income tax. He had merely said he would consider one in response to a question about a controversial comment Republican presidential candidate Mitt Romney made about “47 percent” of Americans not paying income taxes.

Several times during the debate, he laid out his specific plan for fixing the nation’s fiscal woes — one that did not involve such a tax.

Democrat Tim Kaine, left, and Republican George Allen take part in their first televised debate at the Capital One Conference Center on Thursday in McLean. (Jahi Chikwendiu/WASHINGTON POST)

The faceoff came one day after a Washington Post poll showed Kaine with an eight-point lead in the long-deadlocked contest — among the most competitive races in the country, with the outcome possibly determining which party controls the Senate. With seven weeks to go before Election Day, the race had been neck-and-neck in polls going back to May 2011.

Kaine and Allen are fighting to succeed Sen. James Webb (D), who is retiring from the Democratic-controlled chamber. Allen lost the seat to Webb six years ago.

The debate was mostly civil, touching on issues such as jobs and the economy, women’s issues and gay rights, looming defense cuts and the federal health-care overhaul. Each man seemed to take pains to compliment the other on something; Allen praised Kaine’s response to the Virginia Tech shootings, and Kaine gave Allen kudos for his vigor on the campaign trail.

“Muted” was how Bob Holsworth, a retired Virginia Commonwealth University political scientist, summed up the hour.

But the rhetoric cranked up afterward, when Allen and other Republicans seized on Kaine’s professed openness to a minimum income tax.

Gregory led the program off with a new topic — Romney’s “47 percent” remark — one that has brought headaches to the Republican presidential campaign in recent days. On Thursday, it was Virginia’s Democratic Senate candidate who found himself ensnared in the flap.

The subject might have been expected to cause more trouble for Allen, who presumably would want to distance himself from the sentiment without seeming to criticize his party’s nominee. But it was Kaine who found himself on the defensive as Republicans trumpeted what they called proof of his tax-happy tendencies.

Kaine was responding to a question about Romney’s statement that 47 percent of Americans feel entitled to government help in obtaining housing, health care and food but pay nothing in income taxes. Recapping the comment, Gregory asked Kaine whether he thought too many Virginians were dependent on the government for basic needs.

Kaine called Romney’s comments “condescending and divisive.” But when Gregory pressed him about whether everyone in Virginia should pay something in federal income taxes, Kaine said he would consider such a plan.

“I would be open to a proposal that has some minimum tax level for everyone, but I do insist many of the 47 percent that governor Romney was going after pay a higher percentage of their income in taxes than he does,” Kaine said.

The Republican Party of Virginia’s promptly issued a news release declaring: “On Tax Hikes, Tim Kaine Just Can’t Help Himself.”

As quickly as the GOP made hay with Kaine’s comment, the Democrat sought to clarify his comments after the debate.

“David asked me a question, which is: Would I be open to a discussion about something broader like that? And I said, ‘Sure, I’d be open,’ ” Kaine told reporters. “Shouldn’t be news that somebody wants to go into the Senate as willing to start from a position of openness and a dialogue.”

He also tried to weave his response into a larger narrative about himself as a bipartisan problem-solver who can help break Washington gridlock and about Allen as a partisan idealogue like those preventing Congress from tackling the nation’s staggering budget challenges.

“We can’t start with non-negotiables,” Kaine said. “So when my opponent says we have to solve our problems, but we can never consider any new revenue, even one dollar for every 10 dollars of cuts, or we could never find one dollar of savings on the defense side — you’ve got to start with an openness and not with non-negotiable positions.”

No matter how Kaine’s comment plays for voters, it made for some strange role reversals Thursday.

There was Allen after the debate, suggesting that the people Romney had characterized as moochers shouldn’t have demonstrate personal responsibility by chipping in on income taxes. “I don’t think everyone ought to be paying income taxes,” he said.

And there was Kaine, who’s spent most of the campaign having to fend off super-PAC attacks that he’s too liberal, feeling the need to declare that, as governor, he had taken “tens of thousands off Virginians, low-income Virginians, off the tax rolls.”

Until now, the candidates have waged a fierce but somewhat stale battle, with the campaigns and outside groups pouring millions into TV ads attacking one or the other on issues dating back to their tenures as governor, which ended in 2010 for Kaine and 1998 for Allen. Kaine has also criticized Allen for his performance in the Senate, but even that was six years ago.

Some Democratic attacks on Allen have had a somewhat dated quality, as party news releases seek to remind voters of such things as the 20th anniversary of his vote, as a congressman, against the Family Medical Leave Act.

Allen seemed to walk the requisite fine line in his own response to the “47 percent” question in the debate, managing not to endorse Romney’s comment without dinging Romney himself. When Gregory asked Allen whether he shared Romney’s view of America, Allen would not be pinned down, steering most of his answers to jobs and the economy.

Gregory followed up: “I asked you pointedly: Do you share that vision of America?”

Allen said: “I have my own point of view. People of America still believe in the American Dream.”

The debate, sponsored by the Fairfax County Chamber of Commerce and WRC-TV (Channel 4), was the second of the general election but the first to air on TV. Kaine and Allen debated in Hot Springs, Va., in July. They also squared off in Richmond in December, before Allen had won the GOP nomination. Two debates are scheduled for October.

Errin Haines contributed to this report.