SPRINGFIELD - U.S. Sen. Barack Obama plans to stop in Springfield Saturday for a rally at the Old State Capitol - the same location he announced his campaign for president more than a year ago.

The visit comes amid speculation that Obama could pick a running mate as early as today. The event could be an opportunity for Obama to begin introducing his vice presidential pick to the nation.

The Democratic National Convention opens in Denver Monday. The Republican convention is scheduled begin the week after in Minneapolis-St. Paul, and Republican candidate John McCain is expected to announce his vice presidential pick soon, as well.

Obama spokesman Justin DeJong wouldn't elaborate on the purpose of the Springfield event, other than it marks the beginning of a "battleground state tour" in the days before the convention.

Springfield is where Obama spent the bulk of his legislative career, serving in the Illinois General Assembly. His political mentor, Senate President Emil Jones of Chicago, announced his retirement plans on Monday and joked on the Senate floor Tuesday about becoming Obama's vice president.

The Old State Capitol, where Abraham Lincoln once served, is where Obama stood to announce his campaign on a cold day in February 2007.

Political scientist John Jackson of the Paul Simon Public Policy Center at Southern Illinois University said presidential campaigns often feed on that kind of symbolism.

"This is coming full circle back to Springfield and the Old State Capitol," Jackson said.

McCain plans to focus on states that could be close races in the November election, said state Rep. Dan Brady, a Bloomington Republican helping lead McCain's Illinois push.

Brady said Illinois probably won't likely be visited much by the Republican candidate.

"Senator McCain has not seen Illinois as a battleground state," Brady said.

Obama's Springfield event will be free and open to the public, the campaign said. It's scheduled to happen at the same time and place as a blues and barbecue festival near the Old State Capitol.