U.S. Republican presidential hopeful Mitt Romney is picking up a major endorsement Thursday — that of former U.S. president George H.W. Bush.

A spokesman for Romney says Mr. Bush will announce his backing at an event in of Houston . Mr. Bush's endorsement comes one week after his son, former Florida governor Jeb Bush, pledged his support for Romney's campaign.

Romney also secured a key endorsement Wednesday, when U.S. Senator Marco Rubio of Florida said it is increasingly likely the former Massachusetts governor will win enough delegates to secure the nomination.

In a nationally televised interview, Rubio said he had no doubt that Romney will govern as a conservative if elected, and that he will be far better than incumbent Democratic U.S. President Barack Obama.

The freshman senator, a rising star in the Republican Party and a favorite of the conservative Tea Party movement, is considered by many as a possible vice-presidential running mate. But Rubio said he did not believe Romney will ask him, and that he wants to remain in the U.S. Senate.

Public opinion polls show Mr. Obama's small lead over Romney is increasing, but that American voters continue to be concerned with the economy and rising gasoline prices.

U.S. Representative Steve Chabot, an Ohio Republican, says the focus has been on the various Republican presidential contenders, but that it should turn to President Obama.

“It has been a hard-fought race, no question about that. I would, as a Republican myself, I'd like to see a lot of that firepower that's directed at each other directed at the president's policies. Let's have the American people focus on what he's done right or done wrong.”

Romney is far ahead of the other Republican presidential hopefuls in the number of delegates needed to secure the nomination.

Various news outlets say Romney has as least 560 delegates, while his nearest rival, Rick Santorum, has at least 260 delegates, followed by Newt Gingrich with at least 135. A candidate needs 1,144 delegates to win the nomination to face Mr. Obama in the November general election.