WASHINGTON (CNN) -- One of the nation's most influential evangelical leaders is lashing out at Sen. John McCain, a top tier candidate for the 2008 Republican presidential nomination.

Speaking on a Dallas Christian radio program last week, Dr. James Dobson, founder of Focus on the Family, said he wouldn't support McCain's candidacy "under any circumstances."

"He's not in favor of traditional marriage, and I pray that we won't get stuck with him," Dobson added.

Dobson's comments highlight a major political problem for the Arizona Senator: He remains estranged from his party's core voters -- conservative evangelicals.

McCain does oppose abortion and same sex marriage.

But he's refused to get behind a federal constitutional amendment defining marriage as between a man and a woman. The senator says that's a matter best decided on the state level.

McCain has long had an arm's-length relationship with the Christian right, but he is actively working to build bridges as he eyes a 2008 White House bid. Last spring the senator delivered the commencement address at the Rev. Jerry Falwell's Liberty University, after claiming in the 2000 presidential campaign that Falwell and other Christian leaders were "agents of intolerance."

While Dobson's comments suggest McCain may still have fences to mend with the Christian right, a spokesman for the senator said he is confident McCain's record speaks for itself.

-- CNN's Bob Franken and Claire Brinberg