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

The latest Gallup poll, issued Wednesday, shows Romney had the support of 42 percent of Republican voters, and former U.S. Senator Rick Santorum 24 percent, when Santorum left the race Tuesday.

The other two candidates, Ron Paul and Newt Gringrich, have little chance of overcoming Romney for the Republican nomination. Each received only 10 percent support in Gallup's poll April 4 through 9 of more than 1,000 Republicans and Republican-leaning voters.

The poll results suggest Romney, having lost his closest competitor, is on track to become the party nominee and take on President Obama, a Democrat, in the November general election.

But Romney, a former Massachusetts governor, still has a battle ahead of him. The latest Washington Post-ABC News poll indicates that President Obama is leading Romney 51-to-44 percent, despite lingering concerns about the economy.

Santorum ended his bid for the nomination 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.