WASHINGTON (Reuters) - His campaign is running low on money, he's falling behind in voter surveys and he has begun to aim his biting criticism at the leadership of his own political party.

In Republican Newt Gingrich's roller-coaster presidential campaign, is this a big dip - or the beginning of the end?

Just three weeks after his stunning victory in South Carolina's primary made him the man of the hour in the state-by-state race for the Republican nomination, Gingrich is struggling to remain relevant.

The former U.S. House of Representatives speaker not only has run behind Mitt Romney and Rick Santorum in recent voting contests and polls, he appears to be losing ground in a battle with Santorum to be the anti-Romney candidate some conservatives want so desperately.

The candidates are vying for their party's nomination to face President Barack Obama, a Democrat, in the November 6 election.