Clinton won Massachusetts with 60 percent of the vote, but Obama won the state’s most Democratic district with a 17-point margin. The 2008 Almanac of American Politics calls the 8th, which Obama won with 58 percent of the vote, “by far the most Democratic district in Massachusetts.” It is represented by Rep. Michael Capuano (D).

Obama also won each of the districts represented by the three highest-ranking House Democrats, Speaker Nancy Pelosi (Calif.), Majority Leader Steny Hoyer (Md.) and Majority Whip James Clyburn (S.C.), by more than 10 points.

He said if Clinton fails to win heavily Democratic districts in Texas and Ohio, “we can officially say that Obama’s developed huge momentum.”

Obama’s strong performance in Democratic districts will likely pressure lawmakers backing Clinton to consider switching allegiances or voting for Obama at a brokered convention.

Fourteen House Democrats backing Clinton saw their districts vote for Obama. Obama won the districts of Reps. Jay Inslee (Wash.), Norm Dicks (Wash.), Tammy Baldwin (Wis.), John Lewis (Ga.) and David Scott (Ga.) with more than 65 percent of the vote. All five have endorsed Clinton, and a few have already started to waver. Lewis and Scott have both said they would likely support Obama at a brokered convention.     

However, other Clinton supporters have remained steadfast, even when confronted by a difference of opinion with most of their constituents.

Even though 57 percent of Democratic voters in Rep. Emanuel Cleaver’s (D-Mo.) district voted for Obama, he has said, “I’ll be with Sen. Clinton unless I die before the convention.”

Lawmakers’ support is considered crucial to the outcome of the Democratic primary because political analysts believe there is a good chance that neither Obama nor Clinton will be able to win enough pledged delegates to win the nomination outright before the August convention. That means the 796 superdelegates — Democratic members of Congress, Democratic governors and party officials — could very well cast the deciding votes.

Lawmakers supporting Obama may also face pressure from constituents. Nine House Democrats backing Obama are at odds with their districts. For example, Clinton carried the districts of Reps. Xavier Becerra (D-Calif.) and Linda Sanchez (D-Calif.) with 64 percent and 67 percent of the vote, respectively.     

Clinton has fared better in Republican-leaning districts. That trend may speak favorably of Clinton’s appeal among centrists and conservative Democrats in a general election, Cain said.

In Missouri, which Obama narrowly won, Clinton triumphed in districts represented by Republican Reps. Todd Akin, Sam Graves, Roy Blunt, Jo Ann Emerson and Kenny Hulshof. Clinton’s only victory in Democratic-held territory came in the 4th district, represented by Rep. Ike Skelton (D). But aside from its congressional representative, that part of western Missouri is heavily Republican. Bush beat Kerry by 29 points in the district and topped Gore by 19 points.