In a letter e-mailed to his supporters late Monday night and posted on his campaign website, Santorum says he reached his decision after a face-to-face meeting with Romney last week in Pittsburgh, Pennsylvania. Santorum says he was impressed with Romney's "deep understanding" of the need to promote initiatives he says will preserve the family as "the basic building block of our society."

Santorum also says they agreed that President Barack Obama must be defeated in the November general election "above all else."

Santorum, a former U.S. senator from Pennsylvania, unexpectedly became Romney's chief rival for the nomination after winning 11 states during the primary election season, drawing support from staunch conservatives doubtful of Romney's support for their views. But he dropped out of the race last month after losing several state contests and falling well behind Romney in the number of delegates needed to win the nomination.

Santorum assured his supporters that Romney, a former governor of Massachusetts, shares conservatives' demands for lower taxes and smaller government, as well as their opposition to abortion and same-sex marriage.

Meanwhile, Obama will use a campaign appearance in the northeastern state of New York to pressure congressional lawmakers to approve a number of proposals he says will boost the economy, such as creating new tax credits for companies to develop clean energy and repealing tax incentives for companies that move jobs overseas,

The president will also call for legislation that will help veterans from the wars in Iraq and Afghanistan find jobs as police officers and firefighters.