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.