Former U.S. President George H.W. Bush is planning to endorse Mitt Romney's campaign for the Republican Party's presidential nomination.

A spokesman for Romney says the former president will throw his support to Romney at an event Thursday in of Houston, Texas. Mr. Bush's endorsement comes one week after his son, former Florida Governor Jeb Bush, announced his support for Romney's campaign.

Romney secured another key endorsement Wednesday when U.S. Senator Marco Rubio of Florida said during a nationally televised interview that it is increasingly likely the ex-Massachusetts governor will win enough delegates to secure the nomination.

Rubio said he had no doubt that Romney will govern as a conservative if elected, and that he will be “head and shoulders” better than incumbent Democratic 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.

The latest endorsements from leading Republicans furthers Romney's status as the likely nominee. 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.