Barack Obama finally let the cat out of the bag Saturday in announcing Joe Biden as his vice-presidential pick for the Democratic ticket -- which now turns the focus of speculation back on John McCain and whom he will choose as a running mate.

McCain made a congratulatory phone call Saturday to Biden, his long-time Senate colleague, but the soon-to-be Republican presidential nominee also is building anticipation for his own choice of vice president.

McCain is expected to make the announcement at a rally in Ohio on Friday, the day he turns 72 and one day after the Democratic convention ends, potentially minimizing Obama's post-convention bounce in the polls. The Republican convention will begin the following Monday.

The McCain campaign said Obama's choice of Biden wouldn't affect McCain's decision.

"John McCain is going to make a very principled choice about who he will stand with," McCain spokesman Tucker Bounds told FOX News on Saturday. "It will be someone that is ready to lead, that shares his vision to change Washington and begin to deliver the things that the American people expect."

Another McCain spokesman, Ben Porritt, told FOX News that Obama's choice suggests an attempt to overcompensate for a lack of foreign policy and national security experience.

"We're voting for commander-in-chief, and there is only one," Porritt said. "Either it is going to be John McCain or Barack Obama. This is not a time for a mentor; this is a time for a commander-in-chief."

With many Hillary Clinton supporters expressing disappointment in Obama for not choosing her for the Democratic ticket, McCain could try to capitalize by naming a female VP, such as Alaska Gov. Sarah Palin or former Hewlett-Packard CEO Carly Fiorina.

Most experts, though, consider the leading contenders for McCain's running mate to be former Massachusetts Gov. Mitt Romney, who dropped his presidential bid earlier this year, and Minnesota Gov. Tim Pawlenty.

Several GOP officials said the campaign was preparing for an "unconventional" nominee, an indication that oft-mentioned former Pennsylvania Gov. Tom Ridge, an abortion-rights supporter, or Connecticut Democrat-turned-independent Sen. Joe Lieberman still could be in the running. That category also could include non-politicians whom McCain deeply admires, such as Gen. David Petraeus, the top U.S. commander in Iraq.

Some GOP pundits warned McCain to steer clear of Ridge because of his support for abortion rights. But others have said that McCain could get away with picking Ridge if he reassured social conservatives that his running mate would support his views regardless of personal beliefs.

Romney, who bitterly competed against McCain in the primary, could be viewed as a liability because of his Mormon faith, which might not sit well with Christian conservatives.

Two officials close to Romney said he had not been offered the job. Pawlenty batted away questions Friday in a CNN interview, saying, "I'm sure he'll make a wonderful choice for our party and for our country and we'll just have to wait until next Friday to find out the answer to those questions."

The GOP convention begins Sept. 1 in St. Paul, Min.

FOX News' Serafin Gomez contributed to this report.