The president started his visit on Wednesday with a rally in Denver, where he was introduced by Georgetown University student Sandra Fluke.
She gained notoriety after conservative talk show host Rush Limbaugh called her a slut because she supports the Obama health care law's requirement that insurance companies cover contraception.
"When my opponent was asked if he'd fight to guarantee an equal day's pay for an equal day's work, you know what the campaign said? They said, 'we'll get back to you on that,'" Obama told a largely female crowd of about 4,000.
"That's not a good answer. We'll get back to you on that? He won't say what he'd do about it. You've got my answer. Upholding the principle of equal pay for equal work was the first bill I signed into law."
The president is also visiting Grand Junction, Pueblo and Colorado Springs.
A new Quinnipiac University poll shows Obama and Mitt Romney tied among voters in Colorado households earning between $30,000 and $50,000 per year - an important target.
Obama leads among voters with lower incomes; Romney is favored by those making more.
(Copyright 2012 by The Associated Press. All Rights Reserved.)