Rick Santorum's nationwide lead expanded Monday in Gallup's daily tracking poll, which showed the former Pennsylvania senator leading rival Mitt Romney by ten percentage points.

Santorum had the backing of 36% of registered Republicans nationwide, while 26% supported Romney. Numbers from Sunday showed a slightly smaller lead for Santorum over Romney, 36% to 28%.

Romney has been sinking in Gallup's daily tracking poll since last week, when the survey showed him statistically tied with Santorum, 33% to 31%. Since then, Romney has dropped seven percentage points, while Santorum has risen five points.

Both candidates are currently engaged in bitterly negative campaigning in the next battleground states, Arizona and Michigan, hoping to nab wins in states with large delegate counts. Polls in both states show increasingly tight races between Romney and Santorum.

The two other candidates in the running, former House Speaker Newt Gingrich and Texas Rep. Ron Paul, both polled well below the two frontrunners in Monday's Gallup tracking poll. Gingrich had the support of 13% of Republicans while 11% backed Paul. Those numbers were unchanged from Sunday.

For the ongoing poll, Gallup surveys more than 1,000 registered Republican voters on a daily basis, and the poll drops the oldest of five days each day and adds results from the most recent day of polling.

In the latest survey, Gallup questioned 1,194 Republicans and Republican-leaning independents by telephone February 15-19, with a sampling error of plus or minus three percentage points.