WASHINGTON (Reuters) - Senator Susan Collins is back in the spotlight as a crucial swing vote in the U.S. Senate as she raises questions about how combining a Republican tax-cut plan with a partial repeal of Obamacare will affect middle-class Americans.  A day after Senate Republican leader Mitch McConnell decided to link the two issues in a risky strategy, Collins, a moderate Republican from Maine, was citing data that she called worrisome, casting new doubts over the tax plan’s outlook. She told reporters in the Capitol on Wednesday that her staff’s research showed pairing tax cuts with an effective repeal of the individual mandate of Obamacare, formally known as the Affordable Care Act (ACA), could be a mistake. “I have data that demonstrates for certain middle-income individuals and couples, who do not qualify for subsidies under the ACA ... that the premium increase will outweigh the tax cut that they get,” she said. “I suspected this, based on what I know about insurance markets, but now I have the actual data.” Collins was one of a handful of Republicans who voted in July to block a broader Republican attempt to dismantle Obamacare, former Democratic President Barack Obama’s signature healthcare law. The failure of the final repeal effort, in which Collins was joined in opposition by fellow Republicans John McCain and Lisa Murkowski, was a stinging defeat for President Donald Trump and Republican congressional leaders eager to fulfill their campaign promise to scrap Obamacare. Collins, 64, a senator since 1997, decided last month against running for governor of Maine in favor of staying in the Senate, where her status as a centrist Republican willing to work with Democrats has made her one of the most influential members of Congress. That has become especially obvious in recent months. After her role in halting Obamacare repeal efforts during the summer, she said in September she would oppose another Republican healthcare overhaul, known as Cassidy-Graham, leaving it short of the votes needed to pass. She cited concerns about its proposed cuts to the Medicaid healthcare program for the poor. Collins, along with McCain, Murkowski and Senator Ron Johnson, has emerged in the past 24 hours as pivotal to winning Senate approval of the tax-cut plan, which is backed by the president and is critical to the party’s 2018 electoral prospects as they seek their first major legislative win since Trump took office in January. In an unexpected move, McConnell on Tuesday inserted a proposed individual mandate repeal in the Senate tax plan. The mandate, long opposed by Republicans, requires Americans to have health insurance or pay a federal fine. The rule is meant to ensure enough young, healthy people are in Obamacare to offset the costs of covering sicker and older people. Repealing the mandate, the nonpartisan Congressional Budget Office said last week, would increase the number of Americans without health insurance by 13 million by 2027 and raise average Obamacare marketplace insurance policy premiums by about 10 percent annually over the next decade. That premium increase could cancel out any tax-cut gains some middle-class Americans stand to make under the Senate tax plan, according to the research cited by Collins, who oversaw Maine’s Bureau of Insurance before she came to the Senate. Collins has not taken a stand on the tax plan. “I am going to wait and evaluate what is in the bill,” she said. “I do believe our taxes need to be overhauled. ... I just don’t know why we had to complicate it by bringing up the ACA.” Johnson, in a statement on Wednesday, said: “Neither the House nor Senate bill provide fair treatment, so I do not support either in their current version.” The Wisconsin lawmaker added he would work with his Republican colleagues to produce better legislation. Murkowski, when asked on Wednesday if she backed the tax-cut plan with a mandate repeal, said she was focused on opening Alaska’s Arctic National Wildlife Refuge (ANWR) to oil drilling, a key goal for the Alaska lawmaker. A committee that Murkowski chairs on Wednesday passed a bill to open ANWR to drilling, which is now expected to be attached to the tax legislation. 