Though most of them flew cross-country last week to cast key votes on Iraq, the Senators doubling as White House hopefuls haven’t spent much time in Congress lately — and the chamber is poised to be an even less familiar place to them come December.

As one Republican leadership aide put it: “The December legislative calendar will look like Swiss cheese because we’re going to have to work around the candidates’ schedules.”

In the past month alone, the five Senate presidential candidates each have earned the distinction of missing more votes than they’ve made. Sen. John McCain (R-Ariz.) lapped the field, having voted just three times since Oct. 23 — twice on the narrowly approved nomination of Leslie Southwick to the 5th U.S. Circuit Court of Appeals and once against cloture on the State Children’s Health Insurance Program bill.

The four Democrats have done only marginally better than McCain, with their numbers padded after taking the red-eye after Thursday’s Las Vegas debate to catch a trio of Senate votes Friday morning. Two of those votes centered on funding for the Iraq War, while the third sought to close off debate on the pending farm bill (critical to first-caucus-state Iowans).

Still, the bragging rights for Democrats seem to stop there. Based on information compiled on a washingtonpost.com database, Sen. Chris Dodd (Conn.) cast just five — or 22 percent — of the 23 votes in the past four weeks; Sen. Barack Obama (Ill.) has showed up for six of the 23 roll calls — 26 percent — and Sen. Hillary Rodham Clinton (N.Y.) has made seven, or 30 percent, of the votes.

Top among them is Sen. Joseph Biden (Del.), who has made nine of 23 votes in the past month, or 39 percent.

The phenomenon isn’t surprising for the five Senate White House candidates, who are eyeing the first round of presidential primaries that begin in Iowa in just over a month. The rivals’ calendars are filled with events there, as well as in the remaining early contests like New Hampshire and South Carolina.

On the GOP side, McCain’s absences are less consequential since the minority rarely needs all 49 Senators on deck for key votes. The one recent exception was during the Southwick confirmation — a vote that Minority Whip Trent Lott (R-Miss.) made clear to McCain that he could not miss.

Lott said last week that while he recognizes “when you are running for president, you are running for president,” he does have an understanding with McCain that he will return to D.C. when the margins appear tight. Lott called on McCain to return for votes on the Southwick nomination, which advanced after 62 Senators agreed to cut off debate and proceed to his confirmation.

“If it’s critical, and I can give him a heads-up, I will,” Lott said of McCain.

For Democrats, the absences could be more of an issue, since the quartet can easily make or break the outcome of a bill.

And while Majority Leader Harry Reid (D-Nev.) has said he won’t organize the calendar around the foursome, his spokesman, Jim Manley, acknowledged it is one factor the Leader takes into consideration. But sometimes, Manley said, Reid has had to press ahead to make sure Senate business gets done, including when lawmakers voted to confirm Michael Mukasey as attorney general earlier this month.

On that vote, none of the five presidential contenders showed up.

Manley said “so far it hasn’t been a problem” that the four Democratic Senators have been on the trail. But he added: “Sen. Reid is confident that if he needs their votes, they’ll do everything they can to come back.”

While that may be, Republicans say the Senate’s fall schedule certainly has felt the effects of the front-loaded primaries. They point to votes held late in the day, or pushed back to give Senators time to travel to D.C.

Don Stewart, spokesman for Minority Leader Mitch McConnell (R-Ky.), said it seems clear that the Democratic majority has had to rearrange the schedule to accommodate the candidates, and he anticipates more of the same heading into the end of the year.

Asked if it was becoming problematic, Stewart responded: “It’s not making things move any faster.”

Dodd, for his part, said there’s no particular science to determining which votes he must cast versus those he can forgo for the sake of his 2008 campaign. Indeed, all four Democrats viewed the Iraq spending bills — one of which set a goal for a withdrawal of troops — as critical votes.

“There’s no great formula for it,” Dodd said after Friday’s votes. “I try to make a calculation if it’s going to be a close vote or if there are things I really care about or that require some special attention.”

Democratic leadership sources suggested that no phone calls to the campaigns were necessary for Friday’s votes given the importance of casting votes related to war funding and the farm legislation. On the latter, influential Iowa Sen. Tom Harkin (D) is the chief sponsor — and his approval is key to any Democrat hoping to win over Iowa caucus- goers.

“There’s too much at stake right now,” reminded a senior Democratic aide.