Trump vs. Rubio and Cruz at Houston Republican debate

By Emily Stephenson HOUSTON (Reuters) - Donald Trump takes center stage at a debate in Houston on Thursday as the favorite to win the Republican presidential nomination, with time running out on his remaining rivals to change a race rapidly tilting away from them. Trump, 69, has won three of the first four contests in the nomination fight for the Nov. 8 election to succeed Democratic President Barack Obama. After easily defeating his rivals in Nevada on Tuesday, the New York billionaire businessman is in position for more victories on March 1, when Republican party contests will be held in 11 states on what is dubbed "Super Tuesday." At a CNN-hosted debate at the University of Houston, Trump's rivals will have one of their last best chances to try to derail the blunt-spoken political outsider before Super Tuesday. Whether they can pull it off is an open question. On stage with Trump will be U.S. Senator Marco Rubio of Florida, U.S. Senator Ted Cruz of Texas, Ohio Governor John Kasich and retired neurosurgeon Ben Carson. None has been able to slow Trump's momentum in previous debates. "Trump is on cruise control," said Eric Fehrnstrom, a former senior adviser to 2012 Republican nominee Mitt Romney. He said Trump should ignore his opponents and focus on the key planks in his platform - a border wall to keep out illegal immigrants, a stronger military, defeating Islamic State and fair trade. "It's getting late in the game for everyone else," Fehrnstrom said. "People who are expecting a sudden shift in the direction of the race are deluding themselves. Trump is Goliath, and we've seen enough of the other candidates to know there are no Davids in this field." Even so, while Trump has scored early victories and is well ahead in national opinion polls, he has some ways to go to clinch the party's nomination, which is decided by the number of delegates sent to the July party convention following the state-by-state nominating contests. So far Trump leads the race with 81 delegates, with Cruz and Rubio well behind at 17 apiece. To secure the nomination, a candidate needs 1,237 delegates. Super Tuesday will be critical because there are nearly 600 delegates at stake in Republican races that day. Rubio, 44, has an added incentive to change the makeup of the race. He is scrambling to attract the financial donors who supported one-time establishment favorite Jeb Bush, the former Florida governor and Rubio's one-time mentor, who dropped out of the race after his disappointing finish in South Carolina on Saturday. "BOMBSHELL?" Bush, the son and brother of former presidents, held a conference call with his top donors on Wednesday. A donor on the call said Bush offered effusive thanks for their support but provided no direction on whom they should now help. On Thursday, Tennessee Governor Bill Haslam endorsed Rubio. Predicting that Hillary Clinton, 68, would win the Democratic nomination, Haslam said Rubio would present a youthful face to the American people in the general election campaign. "With Marco standing next to Hillary Clinton on a debate stage, the choice between the future and the past will be clear to every American," Haslam, a popular second-term Republican governor, said in a statement. Cruz, 45, enters the debate under pressure. He must do well in his home state of Texas on Super Tuesday. Recently, he has been accused by his rivals of using negative tactics, including one that led to the resignation of his spokesman, Rick Tyler. Cruz suffered another setback on Thursday when two Texas newspapers, the Fort Worth Star-Telegram and the San Antonio Express-News, endorsed Rubio. "While Rubio’s message is generally optimistic and policy-driven, Trump and Cruz are often boorish, angry and divisive. Trump is not a conservative, just a manipulator of anger and disenfranchisement," said the Express-News, which had previously backed Bush. Cruz had already been passed over for an endorsement by the state's two biggest newspapers, the Dallas Morning News and the Houston Chronicle. Romney, who has not endorsed anyone yet, offered a pathway for attacking Trump, telling Fox News on Wednesday that Trump's tax records were bound to contain material that could be explosive. "The reason I think there is a bombshell in there is because every time he is asked about his taxes, he dodges and delays and says: 'Well we’re working on it,'" Romney said. Trump, whose trademark on the campaign trail has been rapid and aggressive criticism of rivals and critics, tweeted in response: "Mitt Romney, who totally blew an election that should have been won and whose tax returns made him look like a fool, is now playing tough guy." In the Democratic race, Clinton has faced a stiff challenge from the left by U.S. Senator Bernie Sanders of Vermont. Sanders won overwhelmingly in the New Hampshire primary, one of the early nominating contests, and in Iowa finished just barely behind Clinton. The next Democratic contest is on Saturday in South Carolina, where polls predict Clinton will win. (Additional reporting by Eric Walsh and Jon Herskovitz; Writing by Steve Holland and Frances Kerry; Editing by Peter Cooney and Leslie Adler)