Bob Deuell, a conservative family-practice doctor from Greenville, had his heart set on serving in Congress. When his congressman, Ralph Hall, got wind of it, he invited Dr. Deuell to shadow him in Washington for a few days in 1998. He gave a potential opponent or successor an opportunity to see what an average week would be like should he win the seat. “He wore me out,” Dr. Deuell said. Dr. Deuell gave up on his Congressional aspirations, running instead for the Texas Senate and winning a spot there in 2002. Mr. Hall, a Rockwall Republican who will celebrate his 89th birthday between now and the May 29 primary, is still in Congress. He’s been there since 1981. Comparatively speaking, United States Representative Sam Johnson, Republican of Plano, is just a pup. He’s 81, a Vietnam War veteran and a former prisoner of war in what became known as the Hanoi Hilton. When he was in the Texas House, lobbyists joked that you might persuade him to vote for a bill but that he was immune to political threats. Mr. Johnson, elected to Congress in 1990, has competition in this year’s primary, as does Mr. Hall. And like the people running against Mr. Hall, his opponents are running positive campaigns that do not attack the incumbent. They are walking a fine line. They want to be in place, should the positions open up, without looking like political vultures. “I don’t know their motivation,” Dr. Deuell said of the candidates who have opposed Mr. Hall over the last decade. “What I’ve seen is people say they’re only going to run if Ralph’s not running. Then it’s ‘I’m only going to run to get my name out there.’ Then they get running and start believing their own bull, and the next thing you know, they’re calling Ralph a liberal.” Dr. Deuell said most of those candidates not only lost their races but also spoiled their chances of succeeding the incumbent. “They made people mad,” he said. Lou Gigliotti and Steve Clark are back this year. They were among the five challengers to Mr. Hall in the Republican primary two years ago. He whipped them, getting 57.4 percent of the vote to Mr. Clark’s 29.7 percent and Mr. Gigliotti’s 1.5 percent. Everyone seems to be tiptoeing (it’s early), but the age issue is right at the surface. Mr. Gigliotti, a Wylie resident who owns an auto parts company and races cars, went right to the age thing in an e-mail exchange about the contest earlier this year. “As you know, Ralph Hall is 89 years old and will turn 91 in office if he is re-elected,” he wrote. “I have been in Texas for almost 30 years running a successful business and running a national championship race team. The Choice is Clear. Vote for the ‘Past’ or vote for the ‘Future.’ ” Mr. Johnson has two opponents this year. In his last five races, he has run unopposed in the primary only once. In the others, he never got less than 84 percent of the vote, whether he had one opponent or two. Mr. Hall, who switched to the Republican Party after the 2002 election (he was unopposed as a Democrat), has won handily, too. He’s at it again, defying anyone who might wonder if he’s too old for the job. It’s a biennial parlor game in North Texas politics. Will they run again? What’s the best strategy if you want the seat but don’t want to poach, don’t want to offend a popular incumbent? So far, nobody has figured it out. The winner of the Republican primary in Mr. Johnson’s Third Congressional District will face a Libertarian, Chris Claytor, in November. In Mr. Hall’s Fourth Congressional District next door, the winner will face VaLinda Hathcox, a Democrat, and Thomas Griffing, a Libertarian, in the general election. Both districts were drawn to favor Republicans in November. And if past elections are any guide, the two incumbents have the advantage in the primaries. Some younger voters have never seen a ballot without one of those names on it. Older voters have been pulling the lever for these guys for years. They don’t seem to mind the gray hair.