Two Republican senators whose votes would be needed to pass a huge GOP tax bill are suggesting they will oppose the final version of that legislation if party leaders don't address their concerns about it.

Sen. Marco Rubio of Florida tweeted Friday that if a final version of the tax bill being hashed out in Congress weakened a child tax credit, or reduces a corporate tax without strengthening tax breaks for parents, there are "going to be problems."

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However, there was broad skepticism that Rubio would end up actually voting against a final tax bill, given his history of toeing the party line.

But Maine Sen. Susan Collins on Thursday told television station WABI that she would consider reversing her prior vote for the Senate's tax bill if the final version in Congress does not include amendments she had added to the Senate's version. Those amendments relate to deductions for property taxes and medical expenses.

Collins already was under heavy pressure from Obamacare advocates to oppose the final tax bill if it includes, as the Senate's version did, the repeal of the mandate that most Americans have health insurance or pay a fine.

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House Republicans, whose own bill did not include that repeal, want it included in the final version expected to be voted on by both chambers of Congress by the end of December.

Collins has said she has received assurances from Senate Majority Leader Mitch McConnell, R-Ky., that Congress will subsequently pass two other bills before the end of the year that she hopes will mitigate the effects of repealing the Obamacare mandate on individual insurance plan premiums.

But House Speaker Paul Ryan, R-Wis., has refused to make that promise to Collins. So has the White House.

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The Obamacare advocacy group Save My Care on Friday released a new ad asking Collins' constituents to lobby her to oppose the tax bill if it repeals the coverage mandate. The ad says Republican leaders "lied to" Collins.

Republicans, who currently hold 52 seats in the Senate, need at least 50 GOP senators for vote for a final version of the bill for it to pass. Sen. Bob Corker, R-Tenn., voted against the Senate's version of the tax bill, the only Republican to do so, because of concerns it would balloon the deficit.

No Democrat in the Senate is expected to vote for the final version of the tax bill.

Earlier this year, Collins and a handful of other Republican senators doomed GOP efforts to repeal much of Obamacare by opposing a series of health-care bills.

Obamacare advocates, in addition to bringing pressure on her to oppose a tax bill that includes repeal of the coverage mandate, are hoping that at least one other Republican senator who previously opposed the health bills will vote not on such a tax bill.

One such senator is John McCain of Arizona.

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On Thursday, Save My Care launched a series of ads targeting 13 GOP House members who earlier this year opposed a Republican Obamacare repeal bill.

The group asked constituents of that baker's dozen to call their offices and ask them to vote against the tax bill if it repeals the coverage mandate.