The Republican presidential hopefuls are duking it out, two days before Tuesday's primaries in Alabama and Mississippi.
Rick Santorum nudged rival Newt Gingrich to step aside.
Santorum, who decisively won Saturday's caucuses in Kansas, argued that a head-to-head contest between himself and Mitt Romney should "occur sooner rather than later."
For his part, a defiant Gingrich is predicting victories on Tuesday.
He called Romney the weakest Republican front-runner since Gen. Leonard Wood, who ran for the nomination in 1920 but lost on the 10th ballot to Warren Harding.
Losing Alabama and Mississippi would effectively spell the end for Gingrich, who has been banking on an all-Southern strategy.
The former House speaker's lone primary wins have been in South Carolina and Georgia.
A win for Romney in Alabama, where polling shows a tight three-way contest, could all but bring the GOP nominating process to a close.
The former Massachusetts governor has built a substantial delegate lead against his rivals.
But he has yet to win a state in the Deep South, home to the Republican Party's most conservative voters.