Herb Robinson, a long-shot candidate in the Democratic primary to take on Sen. Scott Brown (R-Mass.), has dropped out of the race, making him the fifth Democrat to bow out since Elizabeth WarrenElizabeth WarrenSenate panel approves Scott Brown as NZ ambassador Senate confirms Trump's first lower-court nominee The Hill's 12:30 Report MORE entered the primary in September.

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Robinson, an engineer, is expected to run instead for the House seat that Rep. Barney Frank (D-Mass.) will vacate when he retires in 2012, according to multiple reports.

Other Democrats who have already cleared the way for a two-person race between Warren and Brown include state Rep. Thomas Conroy, Bob Massie, businessman Alan Khazei and Newton Mayor Setti Warren (of no relation to Elizabeth).

Democrats Marisa DeFranco and James C. King remain in the primary against Warren.