The election campaigns of U.S. President Barack Obama and presumptive Republican nominee Mitt Romney shifted into general election gear on Wednesday, after Romney's main GOP rival Rick Santorum dropped out of the race a day before.

Talking with Fox News, Romney said Wednesday that "the campaign started yesterday, the general election campaign." He said the long primary battle was borne out of the fact that "there's a large number of people in the Republican party who are extraordinary leaders, including some of those who have run in this last contest with me, and so we'll go through that list and decide who could potentially become a president if that were necessary."

After Santorum's exit, Romney has removed the biggest hurdle in his quest to become the Republican nominee, and the Obama camp wasted no time in unleashing attacks against him.

Obama's re-election campaign released a two-minute Web video depicting some of Romney's most conservative statements from the campaign trail, which are expected to alienate liberal-leaning voters, and make Romney's possible return to political middle ground perilous.

"Expect us to keep holding Romney accountable for the positions he committed to during the primary. There will be no Etch-a-Sketch opportunities this year," said Obama campaign spokesman Ben LaBolt.

The video included some of most conservative and controversial statements made by Romney during the long primary battle in which he tried to win over conservative voters. They included " Corporations are people," "I like to be able to fire people who provide services to me," "Planned Parenthood, we're going to get rid of that." It also emphasized on Romney's statement that he was a "severely conservative Republican governor" during his time in Massachusetts governor's seat.