From CNN:

Mitt Romney is set to officially endorse Pennsylvania Senate candidate Pat Toomey Tuesday in Philadelphia, a move that marks the progress Romney has made in shoring up bonds with some key conservative constituencies.
Just over two years ago, with Romney’s presidential effort in full swing, Toomey raised questions about the former Massachusetts governor’s record on fiscal issues.
“Governor Romney’s economic record contains a mixture of pro-growth accomplishments and some troublesome positions that beg to be explained,” the then-Club for Growth president said in a statement, listing the group’s concerns but giving the presidential candidate his conditional approval.
Still, in February of last year, after it became clear McCain had effectively beaten Romney to capture the GOP presidential nomination, Toomey penned a Wall Street Journal op-ed naming solid economic conservatives he thought the Arizona senator should consider as his running mate. Not on that list: Mitt Romney.
Romney also made effort on behalf of another Northeast Republican Monday night, attending a fundraiser for New Jersey’s GOP gubernatorial candidate, Chris Christie.