Presumptive Republican nominee Mitt Romney was campaigning in Pennsylvania ahead of the state's Tuesday primary with Marco Rubio in tow, the US senator widely discussed as a possible running mate.

Rubio is the latest among potential vice presidential picks to hit the campaign trail with Romney, but the first since the frontrunner's main rival Rick Santorum bowed out of the Republican race two weeks ago.

Romney has already pivoted toward President Barack Obama ahead of the November election, and he told a crowd in Greencastle that Americans needed to help him battle against the cancer that is big government.

"This president has a road that he's traveling down where government gets larger and larger and metastasizes into every aspect of American life," Romney said late Sunday.

Obama, he said, subscribes to "a view that somehow government knows better than free people. I disagree entirely."

Romney became the all-but-certain Republican flagbearer when Santorum, a former senator from Pennsylvania, called it quits earlier this month.

And while the frontrunner was focused on a Romney-Obama showdown, he was making a final sweep through the Keystone State as he seeks to secure the delegates needed to be declared the nominee.

Four other states also vote on Tuesday: Connecticut, Delaware, New York and Rhode Island.

Romney was to attend an event in South Park Township outside Pittsburgh early Monday with former Homeland Security secretary Tom Ridge.