By Steve Holland
WASHINGTON (Reuters) - Mitt Romney is likely to formally seal the Republican presidential nomination on Tuesday with a big victory in Texas that may give him a burst of momentum in his November 6 showdown with President Barack Obama.
Texas has 155 delegates at stake in its Republican primary election and Romney needs less than half of those to put him past the 1,144 threshold needed for the nomination.
Romney campaign officials think he will get enough delegates to put him over the top, a resounding achievement after a long, winding campaign battle that has seen him outlast a series of conservative rivals.
With a close fight brewing with the Democratic incumbent, a Lone Star state win will put a positive spin on the week for Romney, who is holding his own against Obama despite intense attacks against his record as a private equity executive and former Massachusetts governor.
Romney will not be in Texas. Instead, he will be campaigning in Las Vegas on Tuesday with a conservative rival he defeated for the nomination, Newt Gingrich, and real estate tycoon Donald Trump.
A victory will put to rest any lingering suggestion that Romney could face a conservative challenge at the Republican convention in Florida in late August as Gingrich had threatened to do when the race was still close.
"It means he is bullet-proof," said Republican strategist Ron Bonjean.
And it means Romney and his campaign can start to help line up convention speakers and negotiate the party's policy platform with the Republican National Committee.
It also sets up a five-month sprint to the November 6 presidential election that is already well underway with Obama attempting to define Romney as a heartless corporate raider for his work at Bain Capital, a private equity firm.
Romney and the Republicans are running even with Obama and the Democrats in raising money in what should assure an even playing field in television advertising that will be critical in swaying voters in the months ahead.
Romney is keeping his sights trained on Obama's handling of the tepid U.S. economy, a strategy that appears to be helping him in an uphill fight to unseat a sitting president. Polls show voters give Romney the nod over Obama when asked who would be better on the economy.
Romney, who lost the Republican presidential nomination to Senator John McCain in 2008, told Wall Street Journal columnist Peggy Noonan he senses "a much higher degree of anxiety today" over the economy compared to four years ago.
"People (are) much less confident in the security of their job, less confident in the prospects for their children," he said.
The Romney campaign feels heartened that he appears to have withstood an initial round of attacks from the Obama team over Romney's work at Bain, which bought and restructured companies, sometimes resulting in a loss of jobs.
"We feel that the president's message is backfiring and his attacks on free enterprise and Gov. Romney's record in the private sector are embarrassing him and his campaign," said Romney campaign spokesman Ryan Williams.
The Obama camp is using Bain to try to undermine Romney's main argument for why he has the savvy to take on the economy - his business record.
"The job of a president is to lay the foundation for strong and sustainable broad-based growth - not one where a small group of speculators are cashing in on short-term gains. It's to make sure that everybody in this country gets a fair shake," Obama told supporters in Iowa last week Thursday.
(Editing by Alistair Bell and Todd Eastham)