U.S. Republican presidential hopeful Mitt Romney can focus all his attention on President Barack Obama on Wednesday, one day after a rival Republican dropped out of the race, effectively conceding the party nomination to him.

Romney, who is far ahead of the two remaining Republican candidates, is on track to become the party nominee and take on President Obama, a Democrat, in the November general election.

The latest opinion surveys suggest Romney has work to do. The Washington Post-ABC News poll indicates that President Obama is leading Romney, a former Massachusetts governor, 51-to-44 percent, despite lingering concerns about the economy.

President Obama attends a private fundraiser in Washington on Wednesday, while Romney seeks voter support in (the northeastern states of)Connecticut and Rhode Island.

Romney's main rival, former U.S. senator Rick Santorum, ended his bid for the nomination Tuesday, a week after losing three primaries to Romney. He had been looking toward the April 24 primary in his home state of Pennsylvania to give his sagging campaign a boost. However, recent polls have indicated he would lose there.

Santorum made the announcement one day after his young daughter, Bella, was released from a hospital where she was treated for a rare genetic condition. He said her illness caused his family to think about his decision and they are looking forward to spending more time with her.

Speaking in Gettysburg, Pennsylvania, Santorum thanked his supporters and said he beat the odds to win millions of votes and 11 states' nominating contests. Santorum attracted the support of evangelicals and conservatives, but, despite his victories after coming from relative obscurity, he was still far behind Romney in the race for the nomination.