WASHINGTON — A liberal group has launched a seven-figure ad campaign to encourage Republicans to vote against the Senate's tax bill.

First up: Sen. Susan Collins of Maine, who has said that she has deep concerns about the Senate version of the bill, which would slash the corporate tax rate, make temporary changes to individual tax rates, eliminate a series of deductions for individuals and businesses, and end the Obamacare mandate that imposes a fine on people who don't buy health insurance.

The group, Not One Penny, began airing its first ad in Maine on Saturday.

The group, which plans to run similar spots in a set of states it hasn't yet identified, has committed to fighting any legislation that cuts taxes for the wealthiest Americans. Both the House-passed version of the tax bill and a Senate companion approved by the Finance Committee would do so.

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"The Trump Republican tax plan would leave us lost in the wilderness just to pay for huge tax cuts for the wealthy ... and it cuts access to affordable health care, including coverage for opioid addiction," a narrator says in the ad, which was provided to NBC News in advance of its first airing. "Thankfully, Sen. Susan Collins told us that she’d say no to tax breaks for the wealthiest. Call her and tell her not to lose her way."

Collins told NBC News' Chuck Todd this week that she believes it's a mistake to combine the tax bill with health care policy, and that she's particularly concerned that, for many Maine families, a rise in insurance premiums from the elimination of the mandate "more than cancels out" any benefit from cuts in tax rates.

"I don't think it's a good idea from either a political or a policy perspective," Collins said. Even if Congress enacts a proposal from Sens. Lamar Alexander (R-Tenn.) and Patty Murray (D-Wash.) designed to make fixes to Obamacare, which Collins supports, she's not sure she would vote for the tax bill, she said.

"I’m not going to make a final decision on the tax bill until I look at all of the provisions," Collins said.

The House passed its bill, 227-205, earlier this week, with the support of all but 13 Republicans who voted. Not a single Democrat voted for the plan. One Senate Republican, Ron Johnson of Wisconsin, has said he would vote against his chamber's bill as it is currently written.

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Because Republicans hold just 52 seats in the Senate, they can afford only two defections if Democrats are unanimously opposed to the bill. Under expedited "reconciliation" rules, Republicans need only a bare majority to pass the bill, and Vice President Mike Pence could break a 50-50 tie.

"Senator Collins must choose between supporting a bill that cuts taxes for the wealthy and well-connected at the expense of working families or supporting the health and financial security of Maine's middle class," Tim Hogan, a spokesman for Not One Penny, said in a statement provided to NBC News. "Senator Collins should do what she knows is right."