When one political party tries to influence the other’s presidential primary race, the reasonable assumption is that someone is trying to stop the nomination of a candidate that party does not want to face in November.

That’s why a Democrat-aligned group injecting itself into the Republican campaign by alleging that Fred Thompson lobbied for abortion rights is a pretty good indication the other side thinks he has serious potential.

Thompson has become a force in the GOP polls even though almost half of Republicans and more than half the rest of the electorate say they don’t know enough about him to form an opinion.

In this case his Democratic foes are trying to define him negatively for GOP voters before he can make a good first impression.

That’s why in the last few weeks we have seen a stream of stories that seem aimed at derailing his candidacy. Some were clearly planted in the news media with information from partisans of other candidates or causes. Others may have arisen independently from a news media itching to vet Thompson.

The former Tennessee senator and movie/TV actor had experienced a sharp rise in the polls, making everyone take seriously what only months ago might have seemed a flight of fancy.

Thompson runs a consistent second among Republican voters nationally and in most states. In many southern states he leads GOP front-runner Rudy Giuliani, the former New York mayor.

So far he trails the major Democratic contenders in trial heats, but given voters’ lack of familiarity with him at least some Democrats are worried he could be difficult to beat in November should he win the GOP nomination.

Thompson’s candidacy has reshaped the campaign so much that even before he officially announced, adversaries were seeking to undermine his candidacy.

Most interesting – and perhaps most telling of his potential – was the effort by a Democrat-aligned abortion rights group to tarnish his credentials among social conservatives, who are a large part of the GOP primary electorate.

And don’t forget it is the Democratic National Committee – not his GOP competitors – that is suggesting the Federal Election Commission cite him for campaign law violations because he has allegedly been abusing his “testing the waters” committee status to effectively run for president without having to disclose his contributors and spending.

The National Family Planning and Reproductive Health Association claimed that Thompson had lobbied the White House on its behalf in 1991. Thompson denied the charge, and John Sununu, who was White House chief of staff at the time, backed up Thompson.

Only time will tell which side eventually turns out to be perceived as being truthful by the electorate. But regardless of the truth of the allegation, its presence is remarkable at this stage of the race.

Memory does not produce a similar case where an interest group tied to one of the major political parties sought so overtly to influence the debate and outcome of the other parties’ nomination fight, especially before a candidate even formally announced.

Think how unusual it would be if, for instance, a conservative group opposed to immigration reform that would allow any legalization of those here illegally claimed that Sen. Barack Obama, before he was in the Senate, had been active in anti-Hispanic activities.

In that mythical case it would be Republicans trying to damage Obama’s reputation with a key Democratic voting block, Hispanics – just as the current allegations about Thompson are aimed at planting doubt about him among GOP-leaning Christian conservatives.

Also appearing in the news media in recent weeks has been a report on files that showed Richard Nixon critical of Thompson’s abilities as a Senate investigator during Watergate.

Those files also led the news media to raise questions about whether Thompson showed a previously unknown willingness to cooperate with the White House in a way that might benefit Nixon. Such questions would undermine the benefits to Thompson’s candidacy of his service as a lawyer for the Senate Watergate Committee, where he revealed that Nixon had a secret Oval Office taping system.

Other media reports in recent days also put the spotlight on Thompson’s wife, Jeri, who is 24 years younger than he, and attractive enough that one TV talk show compared her to a stripper.

Presidential politics is a contact sport. The coming months will show whether Thompson can take a hit and keep on fighting.

And, if he is smart, he will look both to the right and the left when he is crossing the street.