Loyalists, operatives and leaders of the two major political parties will convene in Tampa and Charlotte in the coming weeks for the biggest, most high-profile political events of the summer: the nominating conventions for their presidential candidates.

But for dozens of congressional candidates, both Republicans and Democrats, the smart political calculation is to stay away.

Some plead scheduling conflicts with their campaigns or their kids. Others say the meetings are too time-consuming or too far.

The common thread tying the convention-skippers together is that each is locked in a tight race, and some distance from their parties, their nominees and Congress may improve their chances of winning.

Among those staying away from the GOP’s gathering in Tampa is Virginia Senate candidate George Allen, who faces former governor Timothy M. Kaine (D) in one of the nation’s most closely watched races.

“Since we are locked in a close race and can’t be in two places at once, the focus will continue to be listening and meeting with Virginia families, veterans and small-business leaders,” said Allen spokeswoman Emily Davis.

A spokeswoman said Kaine plans to go to his party’s meeting in Charlotte.

But so far, more Democrats than Republicans are opting out of their convention, leading GOP officials to suggest that vulnerable Democrats are trying to avoid President Obama.

Paul Lindsay, spokesman for the National Republican Congressional Committee, joked in an e-mail that “more people were lined up to see today’s matinee of ‘Abraham Lincoln: Vampire Hunter’ ” — a movie with lackluster ticket sales — “than the number of Democrats willing to be seen with President Obama in Charlotte.”

Rep. Nick J. Rahall II (D-W.Va.) is among those who faced a choice. “I’m a Democrat; I don’t forget from whence I come,” he said in an interview. “But I have made plans to campaign hard in my district starting Labor Day and right on through.”

Rahall, who has voiced support for the president, faces a difficult campaign against Republican Rick Snuffer in a state where a Texas prison inmate got 41 percent of the vote in a Democratic primary challenge to Obama.

While Democrats gather in North Carolina the week of Labor Day, Rahall said he will attend rallies with coal workers, meet with constituents at his district offices and give “civic club speeches.” Reflecting the president’s unpopularity, Sen. Joe Manchin (D-W.Va.) and the state’s Democratic governor, Earl Ray Tomblin, also plan to stay home.

Among Senate candidates, the absence of Claire McCaskill (D-Mo.) may be the most notable. She was an early Obama supporter in 2008, but her approval rating is stuck in the 40s, and she’s running dead-even with potential GOP opponents, so she plans to campaign aboard an RV and avoid Charlotte.

On Monday, McCaskill told Missouri reporters that the president supports her decision to hit the trail instead of going to Charlotte: “He thinks it’s the right thing to do. The notion that I would be out hobnobbing with donors at cocktail parties after Labor Day rather than here in Missouri fighting — if the Republicans think I’m that dumb, they’ve got me confused with somebody else.”

Several campaign aides said they would rather preserve time and money than travel across the country, noting that there are fewer opportunities to score headlines with local newspapers and television stations that no longer cover the events. Besides, there’s very little for most candidates to actually do at the conventions.

Mac Abrams, campaign manager for Sen. Dean Heller (R-Nev.), said his boss will go to Florida only if he’s offered a prime-time speaking role: “If not, he has a campaign to run.”

Mike Mikus, a campaign aide to Rep. Mark S. Critz (D-Pa.), said, “Quite frankly, there are no votes to be gotten at the Democratic convention.” Critz plans to campaign across southwestern Pennsylvania that week.

Also, with congressional approval ratings hovering in the teens, and neither party getting more than 50 percent approval in polls, some candidates are not eager to fully embrace their party affiliation.

Former New Mexico congresswoman Heather Wilson, a Republican Senate candidate, doesn’t plan to go to Tampa. A campaign aide said her children’s busy summer schedule is partly to blame — but she’s also in a tight race against Rep. Martin Heinrich (D) in a Democratic-leaning state that Obama carried by 15 percentage points in 2008. Heinrich plans to attend his party’s convention.

Hawaii’s Senate candidates, former governor Linda Lingle (R) and Rep. Mazie K. Hirono (D), don’t plan to go to their party meetings. Same for Montana’s Sen. Jon Tester (D) and his opponent, Rep. Denny Rehberg (R).

Among House candidates, spokesmen for vulnerable Republicans Richard L. Hanna (N.Y.), Charles F. Bass (N.H.) and Joe Walsh (Ill.) didn’t rule out skipping their convention, while an aide to Jon Runyan (R-N.J.) said he plans to stay home to campaign and spend time with his children. Democratic Reps. Jim Matheson (Utah), Kathy Hochul (N.Y.) and William L. Owens (N.Y.) also plan to stay put and campaign in their competitive districts.

Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee, dismissed talk of convention-skippers as “a Beltway parlor game” and suggested that “this election is going to be all about voters’ concerns over the economy and Republican plans to end Medicare, not about who is attending the convention.”

But Rep. Steve Israel (N.Y.), chairman of the Democratic Congressional Campaign Committee, who avoided the 2000 convention to campaign in his Republican-leaning district, last week told a Reuters summit that he wants Democratic candidates in tight races to stay home.

“I don’t care if the president was at 122 percent favorability right now,” Israel said. “A trip to Charlotte may be interesting, but why leave your districts?”

Ben Pershing, Scott Clement and Michael Brandon contributed to this report.