WASHINGTON – A majority of voters consider President Obama’s health care law to be a tax increase, leaving the president to defend an election-year vow not to raise levies on the middle class, according to a Quinnipiac University poll.

By a margin of 55 percent to 36 percent, respondents said the Patient Protection and Affordable Care Act amounts to a tax increase.

Participants were less certain of what they thought about the future of the law. They said 48 percent to 45 percent that the Supreme Court was right to uphold it, but also said 49 percent to 43 percent that Congress should repeal it.

“Obama has worked mightily to avoid the ‘T’ word,” said Peter Brown, of the Hamden, Conn.-based Quinnipiac University Polling Institute, in a statement accompanying the poll, conducted July 1 to 8.

The main question is whether Republicans can convince voters that the health care law “breaks his promise not to raise taxes on those who make less than $250,000,” Brown said.

Results released from the same poll Wednesday found Obama leading former Massachusetts Gov. Mitt Romney, the presumptive GOP nominee, 46 percent to 43 percent, helped by a 2-1 edge among single women.

Most respondents in the poll said the Supreme Court decision won’t affect their vote; 27 percent said it would make them less likely to vote for Obama, and 12 percent said it would make them more likely to back the president.

Overall, 55 percent of voters said a candidate’s position on health care is extremely or very important to them.

Americans are split, 48 percent to 47 percent, on whether everyone should have to carry health insurance, the poll found. Women support the mandate 50 percent to 45 percent; men oppose it by the same margin.