(Image Credit: Jessica Kourkounis/Getty Images)

Vetting a running mate, as Sarah Palin's run for VP showed in 2008, is a critical ordeal  for a party's presidential nominee. But already  two of Mitt Romney's top choices for the bottom half of the ticket are embroiled in emotional social issues in their states that could spill over onto the national stage if they are picked for the GOP ticket.

In New Jersey, Gov. Chris Christie has vowed to veto a gay marriage bill passed Thursday by the state assembly. And in Virginia, Gov. Bob McDonnell might be forced to approve or dismiss a bill that defines life as beginning at conception, not birth.

In both cases, the potential running mates' predicaments underline the dilemma  of being  governor of a state while under consideration for the VP spot on the presidential ticket.

In recent years, more Americans have said they are comfortable with gay marriage. In May, for the first time, a majority of Americans approved of it, according to the Gallup poll. Christie, a conservative who was courted by the right wing of his  party to run for president this year, has staked himself to the right of the mainstream by promising to veto New Jersey's gay marriage bill, but at the same time has won cheers in the conservative camp  for his position.

Christie's office said it would release a statement soon on the governor's planned veto. Steven Goldstein, the CEO of the pro-gay Garden State Equality advocacy group, told ABC News that he expected a veto and has started planning how to get two-thirds of each of New Jersey's two legislative houses to approve an "override" by January 2014.

"It's not easy to pass a bill with a hostile governor," Goldstein said.

McDonnell's situation in Virginia is murkier. The state's House of Delegates passed a bill that gives a fetus the same rights as a grown person, defining conception as the beginning of life, a key distinction for opponents of abortion rights. If the State Senate approves it, the bill goes to McDonnell.

McDonnell's office said  he hasn't made up his mind on what he would do yet. "He has not made a decision," said Jeff Caldwell, McDonnell's press secretary. "He'll look at it if and when it passes the General Assembly."

Asked if McDonnell has read the bill, Caldwell said, "He's heard about it."

Signing the bill would surely please conservatives while possibly diminishing McDonnell's appeal among independents on a ticket with Romney.

The head of the Republican Governors Association, McDonnell spoke at an annual gathering of conservatives in Washington last week, along with another top VP option, Sen. Marco Rubio. Christie didn't speak.

At the recentlyconcluded CPAC conference, the conservatives indicated in a straw poll that Rubio was overwhelmingly their  top choice for VP.  McDonnell and Christie came in a distant second.