According to research by tormala and petty an attitude can be

The answer is "Attitude can be strengthened by a persuasive message or argument against it".

Zakaray L. Tormala and Richard e. Petty, both are professors from different universities, Tormala from Indiana University and Petty from Ohio state university. They both have a recent research in 2002 that is about Source Credibility and Attitude Certainty. According to their when people resist inducement, they can distinguish this resistance and become more sure of their initial approaches.