Vice presidential candidates rarely prove decisive in elections, he pointed out. They "mostly have an impact when they are distractions."
"The tabloid nature of these questions have the potential to make them a major distraction unless it's shut down quickly," Kohut said.
The last time a little-known running mate caused a similar stir was 20 years ago, when questions about Dan Quayle's service in the Indiana National Guard were raised after George H.W. Bush sprang his pick on the Republican convention in New Orleans.
Democratic nominee Barack Obama, at a news conference in Michigan, had a sharp reaction when asked if his campaign or other Democrats had been spreading rumors about Palin.
"Our people were not involved in any way in this and they will not be," Obama said.
The pregnancy "has no relevance to Governor Palin's performance as a governor or her potential performance as a vice president," added Obama, pointing out that "my mother had me when she was 18. How a family deals with issues and teenage children - that shouldn't be the topic of our politics."
Joe Trippi, a Democratic strategist from Maryland, said he didn't think the news would hurt McCain.
"If you'd told me she had an abortion, that would be a very devastating blow," he said. "I just don't see how this has any impact on the ticket at all."
In their statement, the Palins said their daughter Bristol had "come to us with news that as parents we knew would make her grow up faster that we had ever planned. We're proud of Bristol's decision to have her baby and even prouder to become grandparents."
The Palins went on to say that "Bristol and the young man she will marry are going to realize very quickly the difficulties of raising a child, which is why they will have the love and support of our entire family."
According to the McCain campaign, the baby is due in late December. The timing is significant, because of rumors that had spread about the youngest member of the family, born 4 1/2 months ago.
Last spring, the Palins issued a statement which said that the governor had delivered a baby at 6:30 a.m. Friday, April 18. She had been attending a Republican governors conference in Texas when she began feeling contractions and leaked amniotic fluid.
The governor told reporters the next week that she phoned her family doctor about 1 a.m. April 17, Alaska time and decided to go ahead with a luncheon speech that day in Dallas, according to news reports at the time.
