Voters in the second most populous US state cast their ballots Tuesday, with Romney the only Republican still actively campaigning for the nomination to challenge Obama in November.
An all-but-assured Texas victory will secure him the 1,144 delegates needed to become the party's 2012 flag bearer, capping a rough-and-tumble slog through dozens of state primaries and caucuses in which he had to overcome doubts about his ideological purity
But while Romney was expected to be celebrating his achievement, the campaign was veering off message thanks to interventions by Donald Trump.
The billionaire developer endorsed Romney in February and spent much of Tuesday fanning once again the flames of a row over the president's birthplace.
Romney was due to appear at a fundraising event in Las Vegas with Trump, who insisted there were still lingering doubts about whether Obama was really a natural born US citizen and therefore eligible to be president.
That provided an opening for Obama's campaign to slam Romney for lacking "moral leadership" over his appearance with Trump.
"If Mitt Romney lacks the backbone to stand up to a charlatan like Donald Trump because he's so concerned about lining his campaign's pockets, what does that say about the kind of president he would be?" said Obama's deputy campaign manager Stephanie Cutter.
Romney's campaign was forced into awkward damage control hours before the two men appear together, with spokeswoman Andrea Saul saying Romney "has said repeatedly that he believes President Obama was born in the United States.
"The Democrats can talk about Donald Trump all they want -- Mitt Romney is going to talk about jobs and how we can get our economy moving again."
Romney did just that at two campaign events Tuesday.
"I'm in this race because I believe in America's greatness," Romney told a crowd in Craig, Colorado.
"Every recession ultimately comes to an end, but you'd expect this deep recession to come back to an aggressive turnaround, and it didn't happen. This president's policies made it harder for America to get on its feet again."
In nominating a multimillionaire former businessman, the Republican Party is in familiar territory, but in one key respect Romney is making history, as the nation's first-ever Mormon nominee of a major political party.
The Republican base has long been dominated by evangelical Christians, and Romney's faith has occasionally come under scrutiny by some religious leaders.
But Romney is counting on Americans seeing him as the pragmatic problem solver with the business credentials to turn the economy around better than Obama has.
Romney, 65, pivoted toward Obama in his campaign speeches and events more than a month ago, when it became clear his long march toward the nomination at the party convention in August would not be stopped.
But it was a brutal primary season. Rivals like Newt Gingrich and Rick Santorum humbled Romney by stealing some victories, rallying voters to their more conservative agenda and highlighting his flipflops on key issues such as abortion.
They also accused Romney of being the wrong candidate to challenge Obama on his historic health care reform, widely reviled by core conservatives, because as governor of Massachusetts he implemented similar reforms that the White House readily states formed the basis for the Obama plan.
But the pugnacious and well-funded Romney always bounced back, unleashing furious assaults on his challengers such as the one in Florida that helped him trounce former House speaker Gingrich, who had alarmed the Romney campaign by winning the South Carolina primary the week before.
In Texas, 155 delegates are at stake. Romney's tally currently stands at 1,064, according to the website RealClearPolitics, some 80 shy of the nomination threshold.
But in Craig and Las Vegas he made no mention of reaching that milestone, and his Tuesday plans do not include the Lone Star State.
Polls show a steadily tightening White House race, with Republicans coalescing behind Romney in the weeks since Gingrich and Santorum dropped out.
Poll aggregates show Obama narrowly ahead. The latest RealClearPolitics average shows the president with a two-point lead, 45.6 to 43.6 percent.
