(FOXNEWS) — Mitt Romney’s Republican rivals attacked his record as a private businessman and challenged him to release his federal income tax returns Monday night in the first of two debates before a pivotal weekend primary in South Carolina.

“I’m proud of my record,” the former Massachusetts governor responded, but he avoided an answer on making his tax returns public.

The debate unfolded hours after former Utah Gov. Jon Huntsman withdrew from the race and endorsed Romney.

That withdrawal raised the stakes of the debate — and one on Thursday night — for Newt Gingrich, Rick Santorum, Ron Paul and Rick Perry.

Gingrich and Perry led the assault against Romney’s record at Bain Capital, a venture capital firm that bought companies and sought to remake them into more competitive enterprises.

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