Former Florida Speaker Marco Rubio (R) announced in an e-mail to supporters Tuesday afternoon that his campaign raised $340,000 during the second quarter for his Senate bid.Rubio is a darling of many social and fiscal conservatives, but his second-quarter fundraising total only serves to underscore the fact that he is the underdog in the Republican primary against Gov. Charlie Crist, who has the backing of the National Republican Senatorial Committee and most of Washington’s Republican establishmentCrist raised $3 million in just six weeks since announcing his candidacy, according to a report last week in the Orlando Sentinel.On Monday, Crist picked up the endorsement of Florida GOP Reps. Lincoln Diaz-Balart and Mario Diaz-Balart. He also has the support of retiring Sen. Mel Martinez (R-Fla.), whom he and Rubio are hoping to replace next year.As Rubio’s campaign continues to attack Crist from the right, the former state Speaker touted his own endorsements from well-known conservatives in Florida and around the country.“In just the last couple of weeks, Sen. Jim DeMint (R-SC), former Arkansas Gov. Mike Huckabee and Congressman Jeff Miller (Fla.) have joined you in standing strong with this campaign,— Rubio wrote in his letter on Tuesday. “They recognize what you and Americans across our state and country understand — that the strength of this campaign is our firm commitment to stand for conservative ideas and principles.—