WASHINGTON – Sen. Olympia J. Snowe, R-Maine, doesn’t come across as a rebel. But her recent vote with the Democrats in the Senate to slash President Bush’s proposed tax cut – to $350 billion from $726 billion – represents a heretical move for a Republican in a town dominated by a GOP-led White House, Senate and House.

Rarely is one lawmaker so critical to the outcome of a high-visibility, high-stakes policy. But Snowe, an unassuming New Englander with a prized seat on the Finance Committee, is the focus of fierce lobbying from all sides this spring, as House and Senate members wrangle over the eventual size and shape of a new 10-year tax cut. Because she supported Bush’s tax-cut plans in the past, she is considered the most vulnerable to pressure.

Snowe’s decision to join forces with Sen. George V. Voinovich, R-Ohio, and Democrats on the budget resolution – which would limit the cut to $350 billion – proved there is still some fight in GOP moderates, whose influence has waned steadily in recent years. Snowe portrayed her action as simply an effort to broker a compromise; many Democrats wanted no tax cut.

“Obviously it’s not an easy process, particularly given the ferocity of partisanship within the institution itself,” she said in an interview this week. “I try to bridge the difference on issues that are important to the country.”

While Snowe talks in terms of consensus and moderation, those who work with her say she is willing to defy her leaders if she feels it serves a broader interest, whether it’s Maine residents or a larger constituency.

Part of this stems from her upbringing. Orphaned at 9, Snowe was somewhat on her own growing up. She commuted by train by herself from Lewiston, Maine, to a Greek school in downstate New York from age 11. At one point she had to spend hours convincing a police officer at the Grand Central Terminal that she and another student were not runaways.

Snowe, 56, was a widow at 26. She’s now married to former Maine Gov. John R. McKernan Jr.

Sen. Lincoln D. Chafee, R-R.I., remembers one of the first times he approached the Senate well for a vote, when GOP leaders were pressuring Snowe and other moderates “to adhere to the party doctrine.”

“There were senators around her and she said, ‘I know how I’m going to vote,’ in a strong, steely voice. That was impressive,” Chafee said. “I remember her resolve. She had made up her mind.”

Snowe’s determination reflects political savvy as well as her own thinking, since Maine voters demand independence from their elected officials. During her first campaign for the Senate, for example, Snowe handily defeated Democratic Rep. Tom Andrews by highlighting her moderate credentials in the suburbs of Portland while touting her opposition to gun control in the more conservative northern part of the state.

“It very much reflects Maine,” said Sen. Susan Collins, R-Maine, a fellow moderate. “They do not want a senator who blindly adheres to the party line.”

Snowe, like Collins, has broken with her party on issues such as impeachment and abortion. But when Bush pushed through his $1.6 trillion tax cut in 2001, Snowe got behind it, even though she had expressed reservations about its impact on the federal budget.

“Any time we can bring down the burden of taxes I think we should,” Snowe said.

Looking back at the 2001 tax vote, Snowe said the $5.6 trillion budget surplus at the time, plus the fact that Bush lobbied personally to make sure negotiators included a refundable child tax credit, made her feel comfortable about supporting the tax cut.

But now that the country must finance a war against Iraq in the midst of a recession, Snowe said lawmakers must exercise caution at the same time they try to stimulate the economy. A 10-year tax cut of $350 billion is an acceptable compromise, she said.

Still, her move has angered some GOP colleagues, conservative editorial writers and even the president himself. Sen. Jon Kyl, R-Ariz., said he wishes “we could all be supportive of the president at this point.” A recent Wall Street Journal editorial branded her, along with Voinovich and Chafee, as “Daschle Republicans.” And Bush and Vice President Dick Cheney called Snowe and Voinovich in for a meeting in the Oval Office last week, in which they unsuccessfully lobbied the senators to accept a bigger tax cut.

The pressure on Snowe probably will intensify in the coming months, as the Finance Committee begins to hammer out details of any tax cut. The Republicans have a one-seat margin on the panel, meaning a defection by Snowe would give Democrats the vote they need to impose their agenda on the committee.

Thomas Mann, a senior fellow at the Brookings Institution, questioned whether Snowe could hold out.

“There’s so much pressure on those moderates to fall into line,” Mann said.

But Sen. John Breaux, D-La., one of the architects of the $350 billion tax-cut compromise, said the Senate’s centrists were “emboldened” by their recent victory. “A lot of people were not able to resist the pressure she was under, with calls from the president and the vice president,” he said. “By sticking together, it shows how you can be vital and influential.”

Snowe says she will continue her crusade to persuade her peers that she represents mainstream, rather than marginal, Republican voters.

“Frankly, I think I’ve maintained consistency with traditional Republican principles throughout my career,” she said. “I don’t understand why I should be on the outskirts.”