Former Massachusetts Gov. Mitt Romney talks to voters at a stop in Nashville, Tennessee, on Monday.
Recent polls show McCain with a comfortable edge heading into Tuesday's 21 Republican contests.
Romney, the former Massachusetts governor, has been telling voters that he is the conservative alternative to McCain.
Romney has been trying to tap into anger at McCain, reminding voters that the Arizona senator voted against tax cuts and supported campaign finance reform.
"We disagree on a number of issues, and if you look at Sen. McCain's position on a number of issues, you have a hard time distinguishing him from Hillary Clinton and Barack Obama," he said Monday.
Romney, along with former Arkansas Gov. Mike Huckabee, is campaigning in the Southeast before making a last-minute stop in California, where McCain and Romney are neck-and-neck.
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