Steven Senne, Associated Press
Republican presidential candidate, former Massachusetts Gov. Mitt Romney greets people during a campaign stop at a Cousins Subs fast food restaurant, in Waukesha, Wis., Tuesday, April 3, 2012.

WASHINGTON — Mitt Romney has won the Maryland Republican primary, tightening his grip on the GOP presidential nomination against fading rival Rick Santorum.

Romney also was looking for victories later Tuesday in Washington, D.C., and Wisconsin. The former Massachusetts governor is heavily favored to clinch the GOP nomination by June. Santorum has vowed not to abandon his bid despite a string of losses. Newt Gingrich and Ron Paul also remain in the race. Neither is a threat to Romney.