"He is unresponsive to the will of our people; he operates by command instead of by consensus," Romney told his supporters at a rally here.
"In a second term, he would be unrestrained by the demands of re-election. And if there is one thing we can't afford, it is four years of a Barack Obama with no one to answer to," he said.
With ten states voting Tuesday, the results were mixed and the most important race, Ohio, remained too close to call.
That did not stop Romney from expressing his confidence that he would be the Republican nominee for president.
"I'm not going to let you down. I'm going to get this nomination," he promised the crowd in the state in which he once governed.
Romney also criticized the president for his record bringing down the unemployment rate.
"Eight percent unemployment is not the best America can do, it's just the best this administration can do," Romney said.
The Labor Department said last month that the unemployment rate fell in January to 8.3 percent, the lowest rate since February 2009, the first full month Mr. Obama was in office.