The Senate Democratic primary in Illinois today carries more weight than a single election. The winner will be running to salvage a piece of the Obama presidential legacy -- if a Republican wins the seat in November, Barack Obama would become the first senator-turned-president to lose his former seat to the opposite party.

Of course, there isn't much precedent for this. In 2008, Obama became just the third sitting senator elected president, following John F. Kennedy in 1960 and Warren G. Harding in 1920. Kennedy was replaced by the appointed Benjamin Smith, a Democrat, who safeguarded the seat until Edward M. Kennedy came of age in 1962 and won a special election. Harding's first term in the Senate was up in 1920, and Republican Frank B. Willis successfully ran to replace him.

Two experts on the presidency told RealClearPolitics that the loss of Obama's Senate seat wouldn't significantly harm his legacy on its own -- despite its place in the record books -- but it would hurt his effectiveness in office.

"I don't know if it will affect his legacy, but it certainly will have an effect on his presidency," said Julian Zelizer, a historian at Princeton University. "Symbolically, it will be read as another sign of his weakness."

"The bottom line is that Obama needs to retake the political initiative," said Stephen J. Wayne, a presidential scholar at Georgetown University. "He needs a Democratic nominee who can win and will support his policy priorities; he needs to return to the policy and political offensive. The election of a sympathetic Democratic Senator from Illinois will help."

Unable to recruit a bigger name like Attorney General Lisa Madigan, Democrats will rely on one of three mostly untested candidates: state Treasurer Alexi Giannoulias (whom polls show to be the favorite), former Chicago Inspector General David Hoffman or Cheryle Jackson, president of the Chicago Urban League and a former press secretary for impeached governor Rod Blagojevich.

Expected to emerge victorious in the Republican primary is Rep. Mark Kirk, whose moderate voting record in Congress should help him statewide. Kirk was the national party's first choice to take back the seat Obama won in 2004 following the retirement of Republican Peter Fitzgerald, who served one term.

Fitzgerald is the only Illinois Republican to win a Senate seat since 1978, and Democrats have won the state at the presidential level in every election since 1992. However, the Republican win in last month's special Senate election in Massachusetts proved just about any seat is up for grabs this year.

"I doubt whether the President's legacy will stand or fall on that election," said Wayne.

However, Zelizer noted, Obama's agenda took a hit with the Massachusetts loss, and the loss of his Senate seat "would certainly fall into that story."