Presidential hopeful Barack Obama has won the endorsement of two fellow Democratic senators from the heartland- Ben Nelson, a popular moderate in largely Republican Nebraska and Claire McCaskill from Missouri, 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 country 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 Feb. 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.

The backing from Nelson and McCaskill caps a slew of big-name endorsements for Obama over the past week, including former Democratic presidential nominee John Kerry and Arizona Gov. Janet Napolitano.

McCaskill has praised Obama often and was widely believed to favor the Illinois senator over Clinton.

But the Senate freshman had resisted openly supporting a candidate until now, saying she wanted to preserve working relationships with Senate colleagues.

She said last week that she identifies with the desire for change that Obama supporters have reported to pollsters.

Nelson said Obama’s victory speech after winning Iowa’s Jan. 3 caucuses was an effort to reach out to Democrats, independents and “enlightened Republicans,” and that Obama’s campaign epitomizes what Nelson has tried to do in Washington.

Obama is the “prototype of what we need today,” said Nelson, who served two terms as governor.

Nebraska Democrats will choose a presidential candidate Feb. 9.

Nelson often votes with his GOP colleagues, and in 2005 won praise from President Bush, who called Nelson “a man with whom I can work.”

Republicans hold all statewide offices in Nebraska except Nelson’s seat, and enjoy a heavy majority among voters.