TOPEKA, Kan. - GOP contender Rick Santorum sharpened his attack against rival Mitt Romney today, calling him the "worst offender" and comparing him unfavorably to President Obama for not "telling the truth."

At a rally here in the old train depot, Santorum accused Romney of switching his positions on critical issues, such as health care and climate change, and working to pursue the same goals as President Obama - which he said would rob everyday Americans of their liberties and money.

"We already have one president who doesn't tell the truth to the American people. We don't need another nominated by our party to do the same," Santorum said.

"Gov. Romney reinvents himself for whatever the political occasion calls for," Santorum said.

Santorum called Romney "the worst offender" for creating a health care program in Massachusetts that included an individual mandate and then denying how that program influenced a similar plan pushed by the Obama administration.

Santorum, who's in Kansas, whose caucuses are Saturday, with  important contests in Alabama and Mississippi to follow on Tuesday, further suggested that conservatives would have a better chance of winning the nomination if former Speaker Newt Gingrich would quit the race.

"We have an opportunity to narrow this race down so we can go one-on-one with Gov. Romney," he said.

Standing at the back of the crowd was Kansas Gov. Sam Brownback, who vowed  he would not endorse another candidate after backing Texas Gov. Rick Perry, who dropped out of the race right before the South Carolina primary.