WASHINGTON (Reuters) - Six more U.S. senators signed on to support a sweeping Republican tax bill on Friday, leaving only one known Republican opponent - Bob Corker - and virtually assuring the measure would pass despite Democratic opposition. A vote was expected later Friday. Here are the Republican senators who have been pivotal to the bill’s fate. Corker, a deficit hawk from Tennessee, said on Friday he could not vote for the Senate legislation because of fiscal concerns. In a statement, he said he believed the tax overhaul “could deepen the debt burden on future generations.” Corker had stalled momentum on the tax bill on Thursday by demanding Republicans look for more ways to keep the bill from causing the U.S. deficit to balloon. He said Friday he felt it would have been fairly easy to alter the measure in a way that would have been more fiscally sound, but “unfortunately, it is clear that the (Republican) caucus is in a different place.” However, Corker said he had told President Donald Trump in a phone call Friday that he would take a close look at the final version of the bill, expected to be produced by a House-Senate conference, before deciding how to vote on it. The Joint Committee on Taxation estimated on Thursday that the Republican bill would expand the national debt by $1 trillion over 10 years, far short of assertions by Republicans that the tax cuts would pay for themselves. The moderate senator from Maine announced Friday she will support the tax bill after securing several improvements in the text and getting assurances that other legislation would be advanced to help lower health insurance premiums. Collins dislikes a clause in the bill repealing a fee imposed on people who do not comply with Obamacare’s “individual mandate” to obtain health insurance. She said she worried that repealing this fee would drive up insurance premium costs, canceling out gains from tax cuts that many constituents might get from the bill. She said Senate Majority Leader Mitch McConnell had pledged to help mitigate the effect of the repeal by supporting passage of two other healthcare bills before the end of the year. One would help insurers cover expensive patients; the other would continue Obamacare subsidy payments for low-income people for two years. Collins also got three amendments included in the tax bill, including one to keep the state and local property tax deduction of up to $10,000 from federal income tax. This would mirror legislation already passed by the House of Representatives. The Montana Republican signed on to support the bill Friday after having voiced concerns about its treatment of “Main Street” businesses. He said he had won more tax relief for non-corporate pass-through businesses, which include partnerships, sole proprietorships and other non-corporate enterprises. The senator from Wisconsin endorsed the bill after  demanding more for pass-throughs. The bill now features a 23 percent tax deduction for such business owners, up from the original 17.4 percent, said statements from Daines and Johnson. The Arizona conservative announced in a statement on Friday he would vote for the bill. He said he had succeeded in eliminating an $85 billion expensing “budget gimmick” and received a firm commitment from Senate leaders and the Trump administration to work on permanent protections for immigrants who were brought to the United States illegally as children.  Oklahoma’s Lankford came aboard on Friday after having questioned whether tax revenues from economic growth would compensate for an expected increase in the national debt. He announced on Friday he was voting for the bill. Moran had earlier been wary of its impact on the debt.     The Arizona maverick and former presidential candidate announced on Thursday that he would back the tax bill. The senator from Alaska will vote for the tax bill, she wrote on Twitter on Wednesday evening. 