By Patricia Zengerle

WASHINGTON (Reuters) - Tim Kaine handed rival George Allen a gift in their closely watched Virginia Senate race on Thursday, when he said unexpectedly he would be "open" to a plan to force all Americans to pay some level of federal income taxes.

Kaine made his comment in a lunchtime debate when he was asked something that should have been easy for him - to weigh in on Republican presidential nominee Mitt Romney's assertion that 47 percent of Americans pay no federal income taxes and feel entitled to government assistance.

"I would be open to a proposal that would have some minimum tax level for everyone," Kaine said, a comment that seemed ideal for use in Allen's advertising ahead of the November 6 election.

Kaine continued by referring to Romney, saying, "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."

The gaffe came just as polls showed Kaine starting to pull away from Allen, a former governor and senator. The two men are vying for the Senate seat now held by retiring Democrat Jim Webb, who narrowly defeated Allen's re-election bid six years ago.

The close race - a Washington Post poll on Thursday gave Kaine an 8-point lead, but most surveys have been closer - is considered one of the keys to whether Democrats will be able to maintain their majority in the Senate.

Allen's campaign leaped on the remark. It issued a news release shortly after the hour-long debate with the video of Kaine, headlined, "Tim Kaine Says He Wants 'Minimum Tax Level for Everyone.'"

Allen sidestepped the question about Romney's remarks, which have prompted other Republicans to distance themselves, by talking about job creation and saying, "I have my own point of view."

(Editing by M.D. Golan)