WASHINGTON, Aug. 22 (Xinhua) -- Rival U.S. presidential hopefuls Barack Obama and John McCain are both expected to announce their running mates in the coming days, with the national conventions of the Democratic and Republican parties around the corner.

    Although speculation regarding their choice is unabated, prior experience and tradition can be used to extract some clues to the possible answers.

    Among the various ways of picking a vice presidential candidate, five are more reliable and common:

    -- Finding someone who appears to have something in common. When Bill Clinton presented Al Gore as his running mate in 1992, they were seen as reflecting each other in many features: young, Southern, baby boomer and moderate, among others, making voters more likely to believe Clinton's claim that he represented a new generation of Democrats.

    -- Finding someone who can help win some key states in the November elections. John Kennedy, a Massachusetts senator who clinched the Democratic presidential candidacy in 1960, picked Texas Senator Lyndon Johnson as his running mate to complement his lack of popularity in the South.

    -- Finding someone who can compensate in some aspects. Joe Lieberman was invited to share the Democratic ticket with Al Gore in 2000 partly due to his strong criticism of Bill Clinton during the Monica Lewinsky scandal, which could help Gore distance himself from the incumbent administration.

    -- Accepting someone for the sake of party unity. It did not take a long or complicated process for John Kerry to pick John Edwards, the No. 2 in the primaries, to be his running mate in 2004, since the party overwhelmingly wanted the then North Carolina senator to be on the team.

    -- Finding someone who can bring excitement to the campaign. Walter Mondale tapped Congresswoman Geraldine Ferraro in 1984 because putting the first woman on a national ticket was apparently "an exciting choice."

    Whatever criteria are followed, the selection of a vice presidential candidate involves a comprehensive and detailed vetting process, to make sure they would be safe from possible attacks and charges by rivals.

    The campaigns' vetting teams, usually composed of lawyers, accountants and other aides, will comb through the tax returns, campaign-finance reports, financial disclosure statements and other personal information of potential vice presidential picks, searching for a hidden time bomb that could derail a presidential campaign.

    For the presidential campaigns this year, issues of ethics and campaign finance are receiving more scrutiny since both Obama and McCain claim to be reformers with more strict ethical standards.