The GOP front-runner and presumed nominee was to have been joined by former Pennsylvania Gov. Tom Ridge at the Consol stop, but a spokesman for the campaign said the nation's first Homeland Security secretary had to cancel because he was sick.
In a 20 minute speech that reprised his standard stump remarks, Mr. Romney assailed the Obama administration on the economy and depicted the president as an enemy of energy development.
Mocking a recent television interview with Obama strategist David Axelrod, Mr. Romney said the advisor had said, "We've got to get off the economic road were on and take a new direction ... I could not agree more."
Mr. Romney argued that while the president has promoted wind and solar energy, his administration has set up roadblocks to the developments of fossil fuels, such as the coal and natural gas interests developed by his host, Consol.
Mr. Romney has regularly vilified President Obama as an enemy of the coal industry.
Among the other Republican officials showing their support for the presumed nominee at the Consol site were Rep. Tim Murphy, who faces a challenge Tuesday from Evan Feinberg, Rep. Mike Kelly, R-Upper St. Clair, and Rep. Mike Turzai, the state House majority leader.
Anticipating Mr. Romney's appearance Monday morning, Democratic campaign staffers noted that as as governor of Massachusetts, he had embraced rules against coal plant emissions such as those for which he now criticizes the administration.
Mr. Romney's cross-state tour came as Pennsylvania voters were preparing to go to the polls in a primary that lost considerable suspense with the decision by Rick Santorum to suspend his campaign.
Mr. Santorum nonetheless remains on the presidential preference ballot, along with Mr. Romney, former House Speaker Newt Gingrich and Texas Rep. Ron Paul. That vote, however, amounts to little more than a beauty contest, as the Republican convention delegates are elected directly and are not bound to any candidate.
Voters will also choose nominees Tuesday in other primaries in New York, Connecticut, Rhode Island and Delaware.
Mr. Romney planned to spend Tuesday evening waiting for the votes in New Hampshire, the site of a vacation home and his first victory on the road to the GOP nomination that he has unofficially captured.
Mr. Rubio has been the object of running mate speculation -- for Mr. Romney and for other GOP contenders -- since the beginning of this election cycle.
Mr. Ridge was once considered a potential vice president himself.
In 1996, when former Sen. Bob Dole was the GOP nominee and again, at least for a time, in 2000, Mr. Ridge's name was frequently mentioned as a potential nominee. In the intervening years, however, the rightward drift of national Republicans on social issues has made the choice of someone with his pro-choice record a political impossibility.