DENVER — Senator McCain will officially unveil his running mate Friday at a large rally in Dayton, Ohio, his campaign has confirmed amid intense speculation over who the soon-to-be Republican presidential nominee had picked.

Top contenders include a former Massachusetts governor, Mitt Romney; Governor Pawlenty of Minnesota, and Senator Lieberman, an independent of Connecticut who caucuses with the Democrats.

By announcing his choice Friday, Mr. McCain is aiming to turn the country’s attention quickly toward his campaign following the weeklong focus on the Democratic National Convention here. There were also reports that the McCain campaign was going to leak the name in the midst of the convention’s climactic night in an effort to distract from Senator Obama’s widely anticipated speech accepting the Democratic nomination before a stadium crowd of 75,000 supporters.

The Republican National Convention begins Monday in St. Paul, Minn., and Mr. McCain will claim his party’s nomination Thursday night. Mr. Pawlenty set off another round of speculation when, after attending a news conference here to criticize Mr. Obama as ill-prepared for the Oval Office, he abruptly canceled an interview with the Associated Press and other appearances to rebut the Democratic convention.

A spokesman for the McCain campaign, Tucker Bounds, yesterday would say only that the pick would be announced “tomorrow in Dayton.” The rally is set to begin at noon, and as many as 10,000 people are expected to attend.

Mr. Pawlenty, 47, is a generation younger than Mr. McCain and is in his second term as governor of Minnesota. An early and loyal supporter, he has urged his party to reach out to what he calls “Sam’s Club Republicans,” working-class voters who are looking for economic security. Mr. Pawlenty is a converted evangelical and is well liked by social conservatives, but he is little known outside his home state, despite the increased exposure as a frequent surrogate for Mr. McCain.

Mr. Romney, 61, has patched up relations with Mr. McCain in the months since the conclusion of their occasionally bitter Republican primary fight. He is said to be favored by advisers to President Bush and has built support among conservatives, despite concerns among some evangelical Christians about his Mormon faith. While most Republicans would likely welcome a Romney pick to help shore up support of the party base and help in states such as Michigan and New Hampshire, Democrats would also welcome seeing him on the ticket. They have a bevy of clips at the ready to remind voters of Mr. Romney’s harsh criticism of Mr. McCain during the primary, and they have had a field day hammering him over his changed positions on key issues, particularly abortion.

Picking Mr. Lieberman, 66, would roil the Republican base and risk a revolt by conservatives at next week’s convention, but it would mark a distinct shift to the political center. The Democrats’ 2000 vice presidential nominee is one of Mr. McCain’s closest allies, but several prominent party elders, including reportedly Karl Rove, have urged Mr. McCain against choosing him.

Republicans last night also left the door open for a surprise pick, including a Texas senator, Kay Bailey Hutchison, or a former chief executive of eBay, Meg Whitman.

Mr. McCain made sure to draw attention to himself in another way last night by airing a television ad in which the Arizona senator, speaking directly to the camera, congratulates Mr. Obama on his historic achievement as the first African American to win the nomination of a major political party.

“Senator Obama, this is truly a good day for America,” Mr. McCain says in the ad, which his campaign said was to air on national cable TV and in key states in the hours surrounding Mr. Obama’s speech. “Too often the achievements of our opponents go unnoticed. So I wanted to stop and say, congratulations. How perfect that your nomination would come on this historic day. Tomorrow, we’ll be back at it. But tonight, senator, job well done.”

Mr. McCain was referring to Mr. Obama accepting the nomination on the 45th anniversary of the Reverend Martin Luther King Jr.’s “I Have a Dream” speech.

Mr. Obama’s campaign manager, David Plouffe, responded in Denver that the ad was “a very nice gesture.”

“We appreciate that,” he said, according to the campaign. “I wish more of his ads had that tone. But for tonight we appreciate it and will congratulate him next week on his nomination.”

The ad is the latest in a series of moves the Republicans have made during the Democratic convention in a bid to grab at least a sliver of the national spotlight and undercut Mr. Obama’s message. They have deployed top surrogates for a daily “Not Ready ’08” press conference focused on Mr. Obama’s political experience. Messrs. Romney and Pawlenty have appeared, as has Mayor Giuliani.

And the McCain campaign has released a group of television ads intended to exploit the lingering division among Democrats and criticize Mr. Obama on national security. One spot focused on Mr. Obama’s decision to bypass Senator Clinton as a running mate, while another featured testimony from a Clinton supporter who had defected to Mr. McCain. A third ad accused Mr. Obama of minimizing the threat of Iran to America and Israel, calling him “dangerously unprepared” for the presidency.

Democrats are planning a similar counteroffensive to Mr. McCain over the next week. Those efforts begin Friday, as Democrats throughout the country will host “More of the Same” birthday parties that will draw attention to Mr. McCain’s age: He turns 72 on Friday.