The choice of longtime Sen. Joe Biden as Sen. Barack Obama’s running mate sends a wide range of signals. Whether those signals harmonize with or are out of tune with the electorate remains to be seen. In what is shaping up as a very close contest between the Democratic ticket and Republican candidate Sen. John McCain, there is little margin for error for the candidates.

Sen. Biden, 65, is a native of important swing-state Pennsylvania, and Democrats are hoping to appeal to voters there with his blue-collar roots. Sen. Biden was first elected to the Senate from Delaware in 1972 at the age of 29. That same year, his wife and one of his three children were killed in a car crash. Sen. Biden remarried five years later. He sought his party’s nomination for president for the 1988 election and again for the 2008 election, failing both times to muster much support in the early primaries.

As a 35-year Senate veteran, Sen. Biden is hardly an external agent of change, which is how Sen. Obama has cast his candidacy. Sen. Obama could have chosen another young outsider, in the same way Bill Clinton defied conventional political logic by running with Al Gore, a fellow baby boomer and southerner.

But with the McCain campaign hitting Sen. Obama early and often on his lack of foreign policy experience, and trying to paint him as weak on defense because of his opposition to the war in Iraq, the Biden selection makes sense politically and practically. Sen. Obama supplements his candidacy with Sen. Biden’s vast experience with foreign policy as chairman of the Senate Foreign Relations Committee. A CNN-Opinion Research poll conducted over the weekend showed 63 percent of registered voters saw Sen. Biden as qualified to be president, with just 26 percent believing him to be unqualified.

Sources close to Sen. Obama told reporters the candidate was more concerned with having a reliable sounding board in the White House than with choosing electoral window dressing. Sen. Biden is certainly not likely to be a “yes man,” and in fact, his biggest liability may be his bluntness and sometimes ill-considered observations.

Within hours of the announcement of Sen. Biden as Sen. Obama’s running mate, the McCain camp began running TV ads showing then-presidential candidate Biden criticizing Sen. Obama’s lack of experience. Those shots likely will not gain traction among voters. But what may be more discordant is Sen. Biden’s support for the invasion of Iraq; as chairman of the Foreign Relations Committee, he helped pass the resolution in 2002 authorizing President Bush to take military action.

Maine GOP Chairman Mark Ellis responded to the selection in a press release: “Barack Obama’s politics of ‘change’ era is over now that he’s chosen longtime Washington insider Joe Biden.” Of course, it’s hard to reconcile that criticism with Sen. McCain’s nearly three decades of service in Congress.

At 65, Sen. Biden likely would be too old to consider a run for president in 2016 if Mr. Obama were to win two terms. Call this the Dick Cheney effect. In fact, the selection of Sen. Biden in some ways mirrors President Bush’s selection of Mr. Cheney, bringing inside Washington experience to a young, inexperienced outsider.

On Friday, Sen. McCain will announce his VP choice, which may provide the most important yardstick by which to measure Sen. Biden.