The rally was held in Craig, a small town in northern Colorado, about 45 minutes from Steamboat Springs.

Romney spoke for about 15 minutes at Alice Pleasant Park, then shook the hands of supporters in the large crowd.

Republican Nomination

Romney is poised to clinch the Republican presidential nomination after Tuesday's Texas GOP primary, a largely uncontested election that will formalize the former Massachusetts governor's status as President Barack Obama's general election challenger.

The Texas primary offers 152 delegates; Romney is just 58 delegates shy of the 1,144 needed to become the nominee.

While Romney's nomination has been virtually assured for a month, the day marks the culmination of several years of work, dating back to his unsuccessful 2008 effort, and perhaps far earlier.

"It'll be a big day tomorrow," Romney told reporters aboard his campaign plane Monday evening. "I'm looking forward to the good news."

Las Vegas Tonight

After his stop in Colorado on Tuesday, Romeny will travel to Las Vegas for a fundraiser with conservative businessman Donald Trump.

The evening event, set for the Trump International Hotel, comes amid fresh criticism from Republicans and Democrats over Trump's continued questioning of Obama's citizenship. Romney hasn't condemned Trump's false claims, offering a fresh example of the presidential contender's reluctance to confront his party's more extreme elements. There have been other examples in recent weeks that underscore Romney's delicate push to win over skeptical conservatives while appealing to moderates and independents who generally deliver general election victories.

Asked Monday to weigh in on Trump's support for the so-called birther movement, Romney declined to condemn Trump's latest suggestion that Obama was born in Kenya.

"I don't agree with all the people who support me. And my guess is they don't all agree with everything I believe in," Romney told reporters before flying from California to Colorado Monday evening. "But I need to get 50.1 percent or more. And I'm appreciative to have the help of a lot of good people."

Trump revived the false claims about Obama's birthplace late last week, citing a discredited story about a literary agency that mistakenly listed that Obama was born in Kenya.

While Romney briefly addressed the issue Monday, his senior aide Eric Fehrnstrom also declined to condemn Trump's remarks in a recent interview.

"I can't speak for Donald Trump ... but I can tell you that Mitt Romney accepts that President Obama was born in the United States," Fehrnstrom said. "He doesn't view the place of his birth as an issue in this campaign."