A constitutional amendment to ban same-sex marriage, backed by President Bush and conservative groups, was soundly defeated in the Senate yesterday after proponents failed to persuade a bare majority of all senators to support the measure. Although most states have acted to prevent same-sex partners from marrying, seven Senate Republicans were wary of wading into the politically risky issue and voted against bringing the proposed amendment to a final vote. ..... Senate Majority Leader Bill Frist (Tenn.) and other GOP leaders had sought the vote as a way to help galvanize their party's conservative base at a time of flagging...