Even a casual reading of the Clinton campaign's email announcing the event to thank supporters (and endorse Obama) demonstrates the trouble Hillary Clinton and her staff are having saying the only words that matter: "I congratulate Senator Obama on winning the Democratic nomination for President of the United States."
The best the writer(s) of the email could manage is, "I will extend my congratulations to Senator Obama and my support for his candidacy."

Nearly every sentence has "I" in the subject position, in one part of the sentence or another.  At the very least, Senator Clinton missed an important opportunity to demonstrate to her supporters that Obama is the candidate that shares the "issues and causes" that matter to them, regardless of their party affiliation. At worst, the email says she is conceding and endorsing while the real message is "I'm still your candidate."
I took a shot at revising the Clinton email to show what a message that actually would indicate support for Obama might look like:

On Saturday, I will hold an event in Washington D.C. to thank everyone who has supported my campaign. Over the course of the last 16 months, I have been privileged and touched to witness the incredible dedication and sacrifice of so many people working for our campaign. Every minute you put into helping us win, every dollar you gave to keep up the fight meant more to me than I can ever possibly tell you.

On Saturday, I will extend my congratulations to Senator Obama on winning the Democratic nomination for President of the United States and offer my whole-hearted support for his candidacy. This has been a long and hard-fought campaign, but the differences between Democrats are small compared to the differences we have with Senator McCain and the Republicans.

I have said throughout the campaign that I would strongly support Senator Obama if he were the Democratic Party's nominee, and I intend to deliver on that promise. On Saturday, I will be asking you to join me in supporting Senator Obama in order to elect our Democratic candidate in November. The stakes are too high and the task before us too important to do otherwise.

Our work isn’t done.  Show the country the same support, strength, and commitment for the Democratic nominee, Senator Obama, that you have shown me in the past 16 months. By supporting Senator Obama, I will keep faith with the issues and causes that are important to you; by supporting him as the Democratic candidate, we can continue to fight together for those issues and causes.

In the past few days, you have shown your support once again with hundreds of thousands of messages to the campaign, and again, I am touched by your thoughtfulness and kindness.

I can never possibly express my gratitude, so let me say simply, thank you.