The Republican presidential candidates are looking for last-minute votes ahead of Tuesday's primaries in Wisconsin, Maryland and D.C.
Polls show Mitt Romney leading in all three races, and a win could put him closer to nabbing the nomination.
Wisconsin representative Paul Ryan is the latest Republican to endorse Romney, but rival Rick Santorum isn't conceding yet.
Santorum would need 74 percent of the remaining delegates to become the nominee. With his campaign faltering, Newt Gingrich's ride could come to an end in Wisconsin.
For the first time, Gingrich is acknowledging that Romney could be the nominee.
Monday, Gingrich will be campaigning in Frederick.
Santorum is counting on a primary victory in his home state of Pennsylvania, set for late April, to keep his campaign in contention.
Meantime, Romney is favored to win Tuesday's primaries in Maryland, D.C. and Wisconsin.