Longtime Clinton associate Bill Richardson's endorsement of Barack Obama last week may be recorded as the tipping point in the Democratic nomination contest. Now, Pennsylvania Senator Bob Casey is endorsing Obama :

Pennsylvania Sen. Bob Casey plans to endorse Sen. Barack Obama for president today in Pittsburgh, sending a message both to the state's primary voters and to undecided superdelegates who might decide the close race for the Democratic presidential nomination.

Dan Pfeiffer, deputy communications director for the Obama campaign, confirmed that Casey would announce his support during a rally at the Soldiers and Sailors Military Museum and Memorial and that he would then set out with the Illinois senator on part of a six-day bus trip across the state.

The endorsement comes as something of a surprise. Casey, a deliberative and cautious politician, had been adamant about remaining neutral until after the April 22 primary. He had said he wanted to help unify the party after the intensifying fight between Obama and Sen. Hillary Rodham Clinton.

Ben Smith calls it the beginning of a bandwagon for Obama:

As readers of Politico Playbook know, Sen. Bob Casey (D-Pa.) -- who had vowed to remain neutral is endorsing Senator Obama today. Democrats are wondering if this could signal the beginning of a "bandwagon effect" that began last week with the endorsement by New Mexico Gov. Bill Richardson.

This is a big break for Obama, who has substantial disadvantages going into the April 22 Pennsylvania primary. Gov. Edward G. Rendell has endorsed Clinton, and the Keystone State is full of the white, blue-collar voters who have turned out strongly for her.

The endorsement also will help Obama with Roman Catholic voters. Casey is one of the state's leading Catholics, and graduated from the Catholic University law school.

I have a feeling that James Carville's smashmouth tactics against Bill Richardson may have helped encourage Bob Casey to get off the fence. It also appears that his kids had some influence on Casey's decision:

Mr. Casey decided to back Mr. Obama because of his "ability to bring disparate groups together and transcend some of these racial and other kinds of divides," the person close to him said. "Also, his kids were on his case, his four daughters. Not that they dictate to him, but he was paying attention. He was wondering, why are these kids, who aren't very political, so interested? He does have the ability to light up a younger generation."

Another aspect of the race that should be changing perceptions among the undecided superdelegates is how Barack Obama dealt with and overcame the controversy surrounding Reverend Wright. It should be clear to all Democrats now that Obama can trade punches with the best of him and that he has good political instincts when it comes to handling the sort of attacks that will come his way in the general election.

In short, there just aren't any good reasons to dump Obama at this point in time, and plenty of good reasons to get behind him.