THE presidency of a Barack Obama or John McCain will look very different if Ted Kennedy is not in the U.S. Senate.

The revelation of the senator's brain tumor on Tuesday will have especially worried Obama, who knows that Kennedy is a critical card for him in November's election. Obama has many reasons for praying for a positive long-term prognosis for one of his most prominent and important supporters.

McCain also knows that when he was searching for a partner on immigration reform, Kennedy withstood the buffeting from many in his own party to stand with McCain on the issue even as colleagues, including Obama, voted in favor of killer amendments to the comprehensive reform bill that went down last year.

Kennedy is that rare political figure who straddles both sides of the aisle in American politics, and has the courage to sometimes defy his own party when he feels a need to.

His importance cannot be overstated. There has not been a major issue in Congress over the past almost half century that Kennedy has not played a key role in.

In recent times he has been especially effective. His stance against the Iraq war from the beginning now looks like the stuff of genius in retrospect.

His embrace of Obama, another shrewd move when the Illinois senator was still in the full flush of battle for the nomination, no doubt gave Obama a credibility among party elders he had lacked.

From health care to immigration reform, both presidential candidates know that without Kennedy at their side during the next administration the prospect of passage is greatly decreased.

Even before that, Kennedy may well play a huge role in the upcoming campaign if he is well enough.

Obama is in deep trouble with Hispanics, for instance, a vital group for him if he is to be competitive in states such as Colorado and New Mexico where his supporters talk of his new majority.

Hillary Clinton however, cleaned up with the Hispanic vote, and the evident tension between Hispanics and blacks makes it a tough hill for Obama to climb come November.

Kennedy, on the other hand, is the darling of Hispanic communities across the states. The link between Kennedy and Hispanics goes back to his brother Bobby's time.

When Bobby began running for president he did so in large part because of the urging of Cesar Chavez, head of the United Farmworkers Union. Without Kennedy as his ambassador Obama will continue to have difficulty with that vital constituency.

When it comes to Ireland and support in the activist Irish American community, Obama would also be heavily reliant on Kennedy. Bluntly speaking, there has been precious little outreach from the Obama camp to Irish Americans. The organizers of the Irish American Presidential Forum have had far greater access to McCain and Clinton personnel than they have had to Obama's staff.

Kennedy is the one politician who could breach the gap there, and the importance of the Catholic vote is such that Obama would be remiss if he did not use him in that capacity.

That, of course, is predicated on Kennedy making a full recovery. The liberal lion may not be able to campaign at all because of his health problems.

Certainly, it's impossible to think that his wife Vicki would approve of any kind of extensive campaign schedule. That will be bad news for the Democratic campaign.

Famously denied the nomination himself in 1980 after a bloody primary battle with Jimmy Carter, Kennedy stood to gain like never before if Obama became president because of his early endorsement.

But it is Obama himself who will most feel the loss of the Kennedy support if it happens through illness. Along with Clinton, Kennedy is the one brand name left in Democratic politics.

The plain fact is that with Ted by his side Obama is a much stronger candidate.