Republican Sen. John McCain (search) is emerging as the bipartisan scold whom neither presidential candidate can get enough of this year.

Although the maverick Arizona senator hasn't been shy about taking both campaigns to task, Democrat John Kerry (search) sounded out McCain as a possible running mate and President Bush will showcase him on the opening night of the Republican National Convention (search).

The phenomenon is explained by the blunt-spoken senator's appeal with voters of both parties, especially among the independents being courted so assiduously by both campaigns this year.

McCain, who unsuccessfully challenged Bush for the GOP nomination in 2000, surfaced at the epicenter of the 2004 campaign again this week as both candidates tried to co-opt his charisma.

The White House announced Thursday that Bush had telephoned McCain to say the president wanted to work with him to rein in political ads being run by outside groups. Happy for any opportunity to put Bush and McCain in the same sentence, White House Press Secretary Scott McClellan volunteered: "We certainly appreciate Senator McCain's strong support for the president, and look forward to hearing his remarks next week at the convention."

Unmentioned by McClellan was the weeklong leadup to the phone conversation, in which McCain called on the White House to condemn outside ads criticizing Kerry's military service. McCain compared the anti-Kerry ads to the tactics that were used against him when he was challenging Bush four years ago.

The Kerry campaign, too, tried to latch on to McCain's magic this week. But it got a nod of disapproval from McCain for running an ad showing four-year-old footage of McCain criticizing Bush for standing by as fringe group questioned his patriotism during the 2000 campaign. Kerry withdrew the ads on Thursday.

McCain, a Navy bomber pilot who spent more than five years as a prisoner of war in Vietnam, is good friends with Kerry. The two worked closely together to end the trade embargo on Vietnam and establish diplomatic relations with the country.

When the Bush campaign this spring tried to suggest Kerry was weak on defense, McCain stomped all over the idea and then chided both parties for waging a "bitter and partisan" campaign.

Earlier this summer, he turned down a request from Kerry to consider being the Democratic nominee's running mate after the two talked over a scenario in which McCain would have had a significant role on defense and other issues.

McCain will be appearing with Bush on Tuesday in Iowa; Kerry can't hope to campaign shoulder-to-shoulder with McCain, but he manages to work frequent references to his fellow senator into his speeches.

McCain emphatically proved his appeal with many groups of voters in 2000 when he trounced Bush in the leadoff New Hampshire presidential primary by a 49-30 percent margin, although his campaign ultimately faded. Exit polls in New Hampshire and elsewhere showed his appeal among voters who wanted someone who stands up for what he believes, and someone who is strong and decisive. McCain also scored well among independents and among Republicans who considered themselves liberal or moderate.

Both Kerry and Bush also know McCain plays well in the media, too. After McCain's 2000 victory in New Hampshire, Bush's brother Marvin told reporters, "That great sucking sound you hear is the sound of the media's lips coming off of John McCain's ..." He cut off the end of his own sentence.