Claire McCaskill campaigning at a diner for the U.S. Senate seat from Missouri that she won in November 2006.

If rumors are true that Barack Obama is considering women other than Hillary Clinton for his running mate, Claire McCaskill must almost certainly be on that list, though she is too savvy not to know that confirming that one is under consideration is a breach of the unwritten rules concerning possible vice-presidential candidates.

The first woman elected to the U.S. Senate from Missouri when she defeated right-wing Republican Jim Talent in November 2006, McCaskill is one of the Democratic presidential nominee’s most prominent female supporters. McCaskill was also one of Obama’s first Senate colleagues to endorse him, and at a time — in mid-January — when more members of the Senate had endorsed Clinton. “This has not been an easy decision for me, McCaskill said at the time, because so many women in Missouri and elsewhere were backing Clinton for the nomination.

McCaskill’s support may have been pivotal in Obama’s victory in the hard-fought Missouri primary, which he won by a margin of only 1.4%. In fact, the contest was so close that the Associated Press initially called Missouri for Clinton, later in the evening having to reverse itself and declare Obama the victor.

Missouri is a medium-size state, but those 11 electoral votes could prove decisive in determining the outcome of the presidential election this year. The Show Me State has gone for the winner in every presidential election since 1956, making it one of the few true bellwether states in the country. If Missouri’s status as a potentially pivotal swing state in November recommends McCaskill as a possible running mate.

McCaskill’s ability to win a closely fought Senate contest in November 2006 against a formidable opponent in a state that is part Northern, part Southern, mildly Midwestern and at the same time Southwestern, further recommends her as a possible vice-presidential nominee. A church-going Roman Catholic, McCaskill could help with that pivotal constituency.

McCaskill just turned 55 on July 24, which makes her eight years old than Obama, who only turned 47 today — a difference in age that could be productive, as the Missourian would be just old enough to give an Obama/McCaskill ticket some ‘gravitas’ without being too old to undermine the contrast with the 72-year-old John McCain.

But if there is much to recommend McCaskill as a potential vice-presidential nominee, there are a few factors that could weigh against her as the putative Democratic nominee considers his choice of running mate.

First, the fact that Missouri has a Republican governor means that McCaskill’s election as vice-president could potentially result in the replacement of the state’s only Democratic U.S. senator with a second Republican to join Christopher ‘Kit’ Bond in the Senate in 2009. Given the importance for a President Obama of having as large a Democratic majority in the Senate as possible, the potential for reducing that Senate majority could prove to be an important factor weighing against the choice of McCaskill as running mate.

Second, McCaskill’s gender could prove as much a disadvantage in the veepstakes as an advantage. For one thing, there are many who believe that the the fact that the putative Democratic presidential nominee is African American (or, to be more precise, biracial) makes for a compelling argument to have a white man on the ticket; those who are of the same mind on this question think that having a person of color as nominee is enough history in the making for the American electorate to absorb in one presidential election.

“While there had been speculation that Obama might seek to mend fences by tapping another woman for the role, this seems increasingly unlikely — and it’s not clear that even if it did happen that it would help with Clinton loyalists, especially since the most-often named women all endorsed Obama in the primaries, earning the resentment of many leaders of women’s organizations,” David Paul Kuhn wrote on Politico.com yesterday.

“If he picked Claire McCaskill or [Janet] Napolitano [or Kathleen] Sebelius, I think it would annoy women,” Geraldine Ferraro is quoted by Kuhn as saying. Not that Ferraro necessarily speaks for all women, but it is an oft-heard sentiment in certain circles.

McCaskill earned a 90% rating for 2007 from Americans for Democratic Action (ADA), which makes her one of the most ‘liberal’ members of the U.S. Senate. Given the importance of the Obama campaign in appealing to independents, moderates, and even Republicans as a different kind of Democrat, McCaskill’s Senate voting record could actually weigh against her in the councils of the senior advisors to the Democratic nominee.

When she ran for the Senate in 2006, McCaskill defied the Missouri Catholic Conference on the issue of embryonic stem cell research, which the Conference adamantly opposes. In 2007, McCaskill got a 100% rating from NARAL Pro-Choice America, 71% from the American Civil Liberties Union (ACLU), and 73% from the League of Conservation Voters. The Sargent Shriver National Center on Poverty Law gave McCaskill a grade of B.

McCaskill’s position on same-sex marriage is essentially the same as Obama’s: she does not support full marriage equality for same-sex couples but instead supports civil unions; and like Obama, McCaskill opposes the federal marriage amendment that would write discrimination into the U.S. Constitution. McCaskill also opposed the 2004 state constitutional amendment that 70% of Missourians voted for which excludes LGBT people from civil marriage under state law.

McCaskill voted for the inclusive federal hate crimes bill, the Local Law Enforcement Hate Crimes Prevention Act of 2007 (also know as the Matthew Shepard Act), which passed the Senate (60-39) in September 2007.

Whether or not the junior senator from Missouri is on the list being kept by the junior senator from illinois, at the very least, McCaskill has become increasingly prominent as an Obama surrogate, as the St. Louis Post-Dispatch has noted. And her ability to offer savvy political advice on reaching out to rural voters in Missouri and elsewhere has made McCaskill a valued advisor in the inner circle of Obama’s most trusted supporters. Even if Obama does not pick McCaskill as his running mate, if the Illinoisan is elected to the highest office in the land, the possibility of a cabinet appointment for his neighbor from Missouri is not at all a distant one.