WASHINGTON (AP) — Newt Gingrich is defending his aggressive attacks on GOP front-runner Mitt Romney, saying the tough questions about his record need to be raised during the Republican primaries.

Speaking Sunday on CBS's "Face the Nation," Gingrich said Republicans must make sure they don't nominate a candidate who collapses in September "under the weight of Barack Obama."

He says the president is expected to spend a billion dollars on his re-election campaign and the GOP has to be able to withstand what he calls the "Obama onslaught."

Gingrich says Romney's record as a former governor of Massachusetts is more moderate than Romney advertises and he needs to "get straight" with voters.

Gingrich says Republicans in South Carolina, which holds its presidential primary next Saturday, would be uncomfortable with Romney as the nominee.