MANCHESTER, New Hampshire (Reuters) - U.S. Republican presidential candidate Newt Gingrich assailed rival Mitt Romney on Friday, accusing him of paying for abortions with tax dollars and raising taxes on blind people.

In a conference call with South Carolina supporters, Gingrich said Romney is a "Massachusetts moderate in the same tradition as Michael Dukakis and John Kerry" and raised "hundreds of millions of dollars" in taxes including on blind people.

Gingrich had pledged to run a positive campaign when he was leading in the polls for the Republican nomination but he has increasingly adopted an angry tone as his lead melted under fire from attack ads, some of them backed by a group close to Romney.

Polls indicate that Gingrich's base of support has plummeted since this week's Iowa caucuses vote, where he came in fourth place.

On the call, Gingrich appealed to the social conservatives of South Carolina by repeatedly accusing Romney of being in favor of gay rights, including taxpayer-funded abortions in the Massachusetts healthcare plan in the form of funding Planned Parenthood, and raising taxes.

"He even raised taxes on people who were blind," Gingrich said, referring to an attempt in Massachusetts to impose a $10 fee for receiving a certificate of blindness.

Referring to next Tuesday's New Hampshire primary and the following nominating contest in South Carolina, Gingrich said the next period is "maybe the most important two weeks in deciding the Republican nomination."

Romney's campaign said Gingrich's attacks were a sign of frustration.

"In a desperate attempt to save his sinking campaign, Speaker Gingrich continues to lash out at his opponents," Romney spokeswoman Andrea Saul said. "Governor Romney is going to keep his focus on the need to fix this economy and put Americans back to work."

(Editing by Eric Walsh)

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