He'll introduce his pick today just hours after Obama's speech to try to retake the momentum.
DAYTON, OHIO - Sen. John McCain will introduce his running mate at a noontime rally today in Dayton, Ohio, kicking off his five-day "Road to the Convention" tour in front of thousands of supporters at Wright State University.
As Republicans begin heading to their four-day convention in St. Paul, McCain kept a tight lid on his selection for vice president and the political calculations that got him there. Aides said they hoped to use today's announcement -- which also is the Arizona senator's 72nd birthday -- to help slow the political momentum from the Democratic convention.
Furious speculation centered on two conservative Republicans: Gov. Tim Pawlenty and former Massachusetts Gov. Mitt Romney. Pawlenty abruptly canceled his schedule Thursday afternoon to fly home on short notice, while reports of Secret Service sweeps of a Romney family member's home in Michigan suggested it was him instead.
A senior Republican operative said there was no evidence of an effort to reach out and soothe conservatives, a move that would probably be vital if McCain picked a candidate who favored abortion rights, such as former Pennsylvania Gov. Tom Ridge or Sen. Joseph Lieberman, I-Conn.
"There's no battle plan to talk to conservatives," the operative said. "That's pretty dispositive in my mind."
Top advisers said late Wednesday night that McCain had made his decision and planned to tell the lucky partner the next day. But in a Pittsburgh radio interview taped Wednesday and made public Thursday, McCain said he had not yet made up his mind.
As the day progressed, the top handful of McCain aides who are privy to the decision went "radio silent," in the words of one top Republican. That left reporters and most Republicans speculating and reading tea leaves.
Early Thursday, McCain advisers teased that they would air an "exciting" and "historic" ad just at the moment that Sen. Barack Obama takes to the stage for his speech. Speculation swirled that McCain might use the airwaves to announce his running mate.
But hours later, the campaign revealed the ad to be a congratulatory statement from McCain. In front of a simple black background, McCain said, "Senator Obama, this is truly a good day for America. Too often the achievements of our opponents go unnoticed. So I wanted to stop and say congratulations. ... Tomorrow, we'll be back at it. But tonight, senator, job well done."
Without real news on a running mate, Internet sites were left to offer screaming headlines with the latest rumors.
Time magazine's "The Page" offered a headline about a TV report that Pawlenty had canceled all appearances, only to follow it with another headline that said he planned to be at the Minnesota State Fair today, not with McCain.
One senior Republican who had talked with Romney, Ridge and Pawlenty during the past two days said, "All of them believe that it's not them."
The Washington Post, New York Times and Los Angeles Times contributed to this report.