Senator Biden is in the mix as a potential running mate for Senator Obama, the Wall Street Journal reported over the weekend. The former governor of Massachusetts, Mitt Romney, is a name that keeps getting mentioned in connection with the vice presidency in a McCain administration. Despite the press speculation, either choice would be something of a surprise, because both the Delaware Democrat and the Massachusetts Republican have significant differences on issues with the candidates who would be at the top of their tickets.

In respect of Mr. Biden, the issue is the procedure known as partial-birth abortion, which Democratic special interest groups see as a fundamental reproductive right that should be left to a woman and her doctor, not Congress. In a May 20, 1997, vote, Mr. Biden sided with Senators Leahy, Moynihan, Daschle, Byrd, and Reid in opposing the procedure. On September 26, 1996, Senators Biden, Leahy, Reid, and Moynihan voted to override President Clinton’s veto of a ban on partial-birth abortions. Messrs. Biden, Moynihan, and Reid had voted for the ban on December 7, 1995. Mr. Biden, along with Moynihan, and Messrs. Byrd, Leahy, and Reid voted for the ban again on October 21, 1999.

So the selection of Mr. Biden as a running mate by Mr. Obama would be a genuine sign that Mr. Obama wanted to unite America and reach out to constituencies, such as pro-lifers, Catholics, and evangelical Christians, who have been neglected by the Democratic Party in recent years. On the other hand, Mr. Obama could expect a backlash from feminist groups if he not only spurns Senator Clinton as a running mate but does so to pick a man who votes against the women’s groups on a key abortion issue.

With Mr. Romney, the issue is immigration. Speaking to the Conservative Political Action Conference in March 2007, Mr. Romney attacked Mr. McCain’s efforts to reach a congressional compromise that included a path to legalization for the millions of illegal immigrants already at America. “McCain-Kennedy isn’t the answer. As governor, I took a very different approach. I authorized our state police to enforce federal immigration laws,” Mr. Romney said then. “I vetoed a tuition break for illegals and said no to driver’s licenses. McCain-Kennedy gives benefits to illegals that would cost taxpayers millions. And, more importantly: Amnesty didn’t work 20 years ago and it won’t work today.”

Both Messrs. Biden and Romney have their selling points. Mr. Biden would add foreign policy heft to Mr. Obama’s inexperience, while Mr. Romney would add a successful capitalist’s understanding of the economy to Mr. McCain’s relative unfamiliarity with economic issues. It just so happens that we share Mr. Biden’s opposition to partial-birth abortion and disagree with Mr. Romney on the merits of the McCain immigration legislation. We were with Mr. McCain.

President Bush’s defenders sometimes portrayed the disagreements within his administration — Secretary Powell versus Secretary Rumsfeld, for example — as evidence that the commander in chief was open to debate on policy and wanted to avoid doctrinaire groupthink. His critics saw the failure to resolve the differences as a sign of weakness that interfered with successful governance. Were the Obama campaign to issue that sort of signal on abortion, or the McCain campaign on immigration, the consequences might be more significant than either campaign might reckon. That will be something for Messrs. Obama and McCain both to ponder as they choose their mates for running.