From CNN's Ashley Killough and Gregory Wallace:

Florida Rep. Connie Mack won Tuesday's Republican primary in Florida over other Republicans seeking to challenge Democratic Sen. Bill Nelson.

Mack's main opponents–former Rep. Dave Weldon and retired Army Col. Mike McCalister–conceded.

The winner of Tuesday's GOP contest will go on to challenge incumbent Nelson in Florida, a crucial swing state in the presidential election.

The Cook Political Report rates the seat as leaning toward the Democratic Party, while another political handicapper, the Rothenberg Political Report, describes the race as a "toss up/tilting Democrat" contest.

Further proof that Obama's statement on same-sex marriage is a contentious issue. The Hill's Alexander Bolton reports:

Senate Democrats facing difficult reelections are breaking with President Obama’s endorsement of same-sex marriage, a sign the issue is politically dangerous in battleground states.

Sens. Jon Tester (Mont.) and Claire McCaskill (Mo.), the two most vulnerable Democratic senators, have declined to endorse Obama’s call for the legalization of gay marriage.

Sens. Joe Manchin (W.Va.), Bob Casey (Pa.) and Bill Nelson (Fla.), Democrats who have easier races but in states that could become more competitive by November, have also backed away from Obama’s stance.

They all represent states with constitutional amendments or laws banning same-sex marriage.