WASHINGTON — Newt Gingrich says Mitt Romney is the weakest GOP front-runner since 1920, but his sizeable campaign fund allows him to run well in races where his rivals don’t have the resources to campaign.

The former House speaker told “Fox News Sunday” that Romney’s money allows him to buy plenty of advertising time that puts him ahead of the pack at first, but his rivals catch up “pretty rapidly” as they campaign.

Gingrich says Romney faces difficulties because the conservative Republican base doesn’t support him.

Gingrich has been campaigning hard in Alabama and Mississippi ahead of Tuesday’s primaries, where he’s hoping to add to the wins he’s had in South Carolina and the state he represented in Congress for 20 years, Georgia.