Presidential hopeful Barack Obama has won the endorsement of two fellow Democratic senators from the Midwest - Ben Nelson, a popular moderate in largely Republican Nebraska and Claire McCaskill from Missouri, a state that is historically a bellwether in presidential contests. Nelson said Saturday he believes Obama has ability to bridge the partisan divide and to carry Democratic candidates across the United States to victory in 2008. Nelson, pledging his support for his Illinois colleague, said Obama has "the greatest potential to ending the bitterness and poisonous atmosphere in Washington." McCaskill plans to announce her support for the Illinois senator Sunday, according to an Obama aide and a McCaskill staffer who spoke Saturday on condition of anonymity so as not to upstage the announcement. Her endorsement is expected to be a major boost for Obama in Missouri, one of nearly two dozen states holding primaries or caucuses on February 5, and could help Obama woo female voters in his race against New York Sen. Hillary Rodham Clinton, his chief rival for the Democratic nomination.