Santorum offered no endorsement in a televised appearance during the evening but said he expected to meet with Romney in the future, adding he would sit down with the former governor's aides on Wednesday.
In his remarks, Romney spoke dismissively of Obama's tenure in office.
"Government is at the center of his vision. It dispenses the benefits, borrows what it cannot take and consumes a greater and greater share of the economy," he said.
He added that if the president's hard-won health care law is fully installed, "government will continue to control half the economy, and we will have effectively ceased to be a free enterprise society."
By contrast, he said, "I see an America with a growing middle class, with rising standards of living. I see children even more successful than their parents."
Romney was eager to leave the nominating campaign behind.
"After 43 primaries and caucuses, many long days and not a few long nights, I can say with confidence — and gratitude — that you have given me a great honor and solemn responsibility," he said.
Romney posed a series of rhetorical questions designed to lead voters to his side.
"Is it easier to make ends meet? Is it easier to sell your home or buy a new one? Have you saved what you needed for retirement?" he asked.
"Are you making more in your job? Do you have a better chance to get a better job? Do you pay less at the pump?"
At each question, his partisan audience shouted, "No."
The nominating campaign that still had some loose ends, including the pursuit of national convention delegates.
Romney is still 300 delegates shy of a nominating majority, although he is far ahead of his most persistent rivals. There were 209 delegates at stake in Tuesday's primaries, and he won at least 146.
That left him with 844 delegates of the 1,144 needed for the nomination, compared with 260 for Santorum, 137 for Newt Gingrich and 79 for Ron Paul.
Santorum suspended his campaign two weeks ago rather than risk losing a primary in his home state of Pennsylvania.
Gingrich, too, seemed to be heading toward the sidelines, although he said he intends to complete his plans for several days of campaigning in North Carolina.
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.