Mitt Romney's four remaining challengers for the Republican presidential nomination did their best to knock the front-runner off stride in a contentious debate, but the best they could do was to get ... more 
Mitt Romney's four remaining challengers for the Republican presidential nomination did their best to knock the front-runner off stride in a contentious debate, but the best they could do was to get him to grudgingly agree to consider releasing his tax returns. less 
1 / 27
WCVB - Boston
Sun, May 6, 2012 9:28 AM EDT