WASHINGTON -- Barack Obama has gained another superdelegate. Minnesota Sen. Amy Klobuchar endorsed her Illinois colleague Sunday night, saying in a statement that Obama "has inspired an enthusiasm and idealism that we have not seen in this country in a long time."

It is the latest development to put Hillary Clinton's presidential campaign on its heels. Obama also got support last week from Pennsylvania Sen. Bob Casey, a key endorsement three weeks before the Keystone State holds its primary.

Klobuchar, a freshman senator, said Obama speaks "with a different voice, bringing a new perspective and inspiring a real excitement from the American people." She compared him to the late Hubert Humphrey, who served as a senator from Minnesota and as vice president under Lyndon Johnson.

Obama carried Minnesota by a large margin over Clinton last month. Klobuchar cited their work together on issues such as ethics reform and toy safety as reasons she handed her endorsement to the Illinois senator.

"Barack has been a proven agent for change and advocate for middle-class Americans," Klobuchar said."My endorsement reflects both Barack's strong support in my state and my own independent judgment about his abilities."

The Klobuchar and Casey endorsements are crucial, because they are superdelegates at the August party convention. Klobuchar's endorsement also tied up the number of senators who have endorsed Obama and Clinton.While Clinton still leads Obama in the superdelegate tally, Clinton has won only nine superdelegate pledges to Obama's 64 since Super Tuesday. Because neither candidate is likely to gain the 2,024 delegates necessary to clinch the nomination before the convention, the candidates have turned toward targeting the 795 superdelegates who aren't bound by state nominating contests. More than 300 are up for grabs still while 550 pledged delegates have yet to vote. Obama has 1,631 committed delegates to Clinton's 1,501.

Democratic National Committee Chairman Howard Dean on Friday called for the remaining undecided superdelegates to make up their minds by July 1 in an effort to avert a convention crisis. His comments came the same day Vermont Sen. Patrick Leahy said Clinton's campaign was a lost cause and she should drop out of the race.

On Monday, The Wall Street Journal had reported that Obama appeared poised to receive the endorsement of the seven Democrats on North Carolina's congressional delegation, The Obama campaign knocked down that story in the afternoon, issuing a statement from the sole North Carolina House member who had endorsed Obama already.

"Senator Obama and his campaign are working very hard to earn the endorsement of my colleagues.  While I would love to have any of them eventually join me in this endorsement, nothing is scheduled at this time, and it is up to each of them to decide when – or if – that will happen," said Rep. GK Butterfield.

The latest Gallup Daily tracking poll shows Clinton ceding more ground nationally to Obama, who now leads 52 percent to 42 percent. The poll, taken March 27-29, included 1,228 Democratic and Democratic-leaning voters and had a margin of error of 3 percent.

The poll was conducted as both Clinton and Obama fought off public criticism -- Clinton for falsely insisting she had landed under sniper fire in a 1996 trip to war-torn Bosnia, and Obama for being a longtime member of the Chicago church headed by the bombastic Rev. Jeremiah Wright Jr.

FOX News' Bonney Kapp and The Associated Press contributed to this report.