When an incumbent politician uses the trappings of office to project an image of power for the purposes of re-election.
The phrase originally referred to a president staying on the grounds of the White House to campaign as opposed to traveling throughout the country. However, it’s taken on a broader meaning in recent years.
For example, the New York Times notes President George H.W. Bush carried out a “Rose Garden strategy” for the 1992 campaign: “Sometimes the strategy puts the President in the Rose Garden, as it did this morning, and sometimes it takes him on the road, as it will to Pennsylvania on Thursday. But it always has one aim: to lift Mr. Bush’s political fortunes by wrapping him in the trappings of his office and having him take steps to demonstrate, as one political aide put it, that ‘he is the man in charge and the others are just wannabe’s.'”