When George Allen fell to Jim Webb in the Virginia Senate race, it opened up a slot in the upcoming Republican presidential primary: the role of the reliable longtime lawmaker who has no serious disagreements with the conservatives who make up the party’s base.

That slot is moving closer to being filled by a former senator of Tennessee, Fred Thompson. The potential candidate is about “50–50” on running “because the polls have caught his eye,” a source close to Mr. Thompson told National Review. The AP suggested this week that a bid by the former “Law and Order” actor would be hindered by “a shrinking pool of campaign professionals” not yet affiliated with GOP candidates.

But a longtime Thompson associate said the former lawmaker has received many calls from veteran Republican campaign staffers expressing interest in working with him if he decides to run. At least one high-level staffer of another Republican presidential candidate has expressed concern about running against Mr. Thompson, citing a long personal connection to him. And last week, Alex Castellanos, a press strategist for a former Massachusetts governor and Republican White House hopeful, Mitt Romney, was seen with Mr. Thompson at a restaurant in Alexandria, Va.

Mr. Thompson’s powerful friends in Tennessee may also help assemble a viable campaign staff. A former Senate majority leader, Bill Frist, once considered his own bid for president but has endorsed Mr. Thompson and would presumably put his network of supporters, fund-raisers, and strategists at Mr. Thompson’s disposal. Another retired Tennessee senator with extensive ties in Republican circles, Howard Baker Jr., also has been nudging Mr. Thompson to run.

One powerful organization that may breathe easier at news of a Thompson bid is the National Rifle Association. Each of the “big three” Republican presidential frontrunners has a glaring flaw in the eyes of the organization, although the group has not sought to emphasize its disagreements with the trio.

As governor, Mr. Romney pledged to continue his state’s “tough gun laws,” which he praised for providing public safety. As a candidate for Senate in 1994, Mr. Romney supported the Brady bill, which imposes a five-day waiting period on gun sales and a ban on certain assault weapons. And as a gubernatorial candidate in 2002, Mr. Romney received a “B” grade from the organization. He only joined the NRA last year, and as of January, he did not own a gun.

For much of Senator McCain’s career, he accumulated a pro-gun record, but the Arizona lawmaker twice co-sponsored legislation that would regulate gun shows more strictly, which the NRA opposed. Furthermore, campaign advertising restrictions enacted under the McCain-Feingold bill further soured the senator’s relationship with the group.

At the NRA’s national convention in 2001, Executive Vice President Wayne LaPierre asked, “Is it possible that John McCain thinks you have too much freedom? … I don’t know what’s happening to John McCain. … Just think — [under McCain-Feingold] eight weeks before a general election, the 4.3 million members of the NRA must shut up and step aside.” (During his Senate career Mr. Thompson was a keen advocate of campaign finance reform, which may offend those conservatives who consider the First Amendment sacrosanct.)

Mayor Giuliani faces the difficult task of overcoming a career-long position favoring various gun control laws. In an early campaign appearance with the radio talk show host Sean Hannity, Mr. Giuliani indicated that he would not support New York City’s gun laws on a national scale: “Yes, I mean, a place like New York that is densely populated or maybe a place that is experiencing a serious crime problem … maybe you have one solution there and in another place, more rural, more suburban, other issues, you have a different set of rules.”

But to the NRA, Mr. Giuliani’s stance means that the Second Amendment can be violated, so long as it is only on the local level — an unacceptable compromise.

By comparison, Mr. Thompson’s relationship with gun-rights groups is sterling. A 2000 report from a campaign-finance watchdog group, Common Cause, found that the NRA, Gun Owners of America, and the Georgia Gun Owners PAC donated $188,954 to Mr. Thompson between 1993 and 1999; the groups donated more only to Dr. Frist. (Mr. Thompson was elected in a special election in 1994.)

Despite often supporting Republican candidates, the NRA insists it is an issue-based organization and not a partisan one. In recent years, the group has endorsed several Democrats, including Governor Bredesen of Tennessee, Governor Easley of North Carolina, and Rep. Marion Berry of Arkansas.

Nonetheless, the endorsement of the NRA Political Victory Fund has correlated with every Republican presidential victory since 1980. In 1992 and 1996, the organization declined to endorse a candidate. After the 2000 election, President Clinton contended in an interview that NRA opposition cost Vice President Gore New Hampshire, Arkansas, and perhaps Tennessee and Missouri.

A former Democratic governor of Virginia, Mark Warner, had some fans within the NRA, but he abruptly lost interest in a presidential bid last fall. While Governor Richardson of New Mexico looks to be a long shot for the Democratic nomination, the thinking within the organization is that it would eagerly endorse a consistently pro-gun Democrat over a Republican who has been inconsistent in protecting Second Amendment rights.