After backing Senate Republicans’ tax bill on Friday, U.S. Sen. Susan Collins signaled Sunday that her party doesn’t necessarily have her support on a final tax product.

The senior senator from Maine said on Sunday on NBC’s “Meet The Press” that she’ll wait to “ see what comes out” of negotiations between Republicans in the Senate and House of Representatives, which is the next step before a final tax roadmap can be approved.

Collins’ support may depend on promises that she has secured from Republican leaders, but they may not be able to make those guarantees. Three Collins amendments were included in the bill that passed the Senate along party lines early Saturday, including a property tax deduction.

The Maine senator has already secured promises from President Donald Trump and Senate Majority Leader Mitch McConnell, R-Kentucky, that two bills aimed at stabilizing the Affordable Care Act will also pass. Collins also has promises from McConnell and House Speaker Paul Ryan, R-Wisconsin, that the bill won’t trigger automatic Medicare cuts.

But the House is a different beast than the Senate and conservatives there have balked at Collins-backed changes to health care law.

The senator was also grilled on debt estimates on Sunday, but she mostly brushed them aside and cited uncertainty. “Meet the Press” moderator Chuck Todd hit Collins with a montage of her past statements of concern about the national debt.

The conservative Tax Foundation says the Senate plan will increase the national debt by $516 billion over 10 years after economic growth. That’s lower than the $1 trillion price tag estimated by the Joint Committee on Taxation.

But Collins says even a relatively small uptick in growth would erase that gap, that she’s talked to other economists who think it would stimulate more growth and that economists “just don’t agree on this.”

In an email Monday, she reiterated her point to Todd that an increase of 4/10s of 1 percent in the gross domestic product would add $1 trillion in revenue to the federal government and that Maine companies such as New Balance and Hussey Seating “have told me this bill will lead to greater investments and new jobs in Maine.”

Her Bangor office was protested by progressives on Monday. Mainers For Accountable Leadership, a progressive group that has protested Collins often in the past, asked people to gather outside her Harlow Street office at 1 p.m., saying in a news release that it is “disappointed” in her vote. To date, there has been no evidence that Collins’ loyal supporters in Maine feel the same way.

Later Monday, a handful of protesters reportedly refused to leave Collins’ Bangor office as an act of civil disobedience to protest her vote, according to a release from Maine labor union organizers.

Meanwhile, the bill’s supporters in Maine — including Maine Republican Party Executive Director Jason Savage — rallied to Collins’ defense. In a Monday interview on WVOM radio, Savage said Collins “made the right choice and she stood up for the middle class Maine people, small businesses, bigger paychecks, better jobs — and we’re really happy about that.”

As the bill progresses through a House-Senate conference committee and further votes, expect the lobbying from both sides to intensify, as opponents of the measure need at least two Republican senators to flip their votes. Collins will likely remain a key target of their efforts.

For a roundup of today’s Maine political news, click here for the Daily Brief. Click here to get Maine’s only newsletter on state politics via email on weekday mornings.

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Michael Shepherd joined the Bangor Daily News in 2015 after time at the Kennebec Journal. He lives in Augusta, graduated from the University of Maine in 2012 and has a master's degree from the University...