Governor Mitt Romney yesterday endorsed a grass-roots effort to pass a constitutional ban on same-sex marriage in Massachusetts in 2008, abandoning his support for what he called a ''muddied" compromise measure that would also ban gay marriages but allow gays to enter into civil unions. Romney, who is courting conservative voters for a possible presidential run in 2008, said the newly proposed ban would give voters a chance to consider a ''clean, straightforward, unambiguous amendment" that does not include civil unions.