As many as a dozen sitting Senators are likely to consider launching campaigns for the White House in 2008, the first time in nearly 80 years there have been two competitive presidential primaries.

The key to unleashing a flood of Senators into both parties’ nominating processes will be the fate of President Bush.

Not since Vice President Charles Dawes chose to follow President Calvin Coolidge into retirement in 1929 have both Republicans and Democrats lacked obvious heirs apparent ready to take the top spots on their parties’ tickets.

In every election since, either a sitting president or vice president has led one of the major national tickets, stunting the ambitions of other politicians eager to explore White House bids. (After President Harry Truman dropped his bid for re-election in 1952, Vice President Alben Barkley’s desire to run was dismissed because of his age.)

But Vice President Cheney is expected to follow Dawes’ path and retire on his own terms in 2009 if Bush wins a second term.

That would open the door to at least a half-dozen ambitious Senate Republicans, a group that is expected to be led by Majority Leader Bill Frist (R-Tenn.) and possibly several members of his leadership team, to seriously explore White House bids.

There is expected to be an equally crowded primary field across the aisle, with at least seven Senators seen as possible candidates for the Democratic nomination.

Throw in a handful of governors weighing bids in 2008, and you’ve got the makings for the most competitive twin nominating contests in nearly eight decades.

“Most Members of the United States Senate” are looking at White House bids, Sen. Larry Craig (R-Idaho) said, unable to suppress a grin. “But I am not one of them.”

So far, no House Member of either party is viewed as a likely candidate in 2008. House Minority Leader Nancy Pelosi (D-Calif.) has said her goal is to become Speaker and makes a point of noting that she has no designs on any other higher office. For Republicans, Speaker Dennis Hastert (Ill.) and House Majority Leader Tom DeLay (Texas), both viewed primarily as creatures of the institution, have also shown no desire to mount national campaigns.

Along with Florida Gov. Jeb Bush, Frist may end up drawing the label of early frontrunner for the GOP’s 2008 nomination should he seek it. The Tennessee heart surgeon has quickly climbed the GOP leadership ladder and was drafted to be Majority Leader in December after Sen. Trent Lott (R-Miss.) was forced to resign the post.

While First has had some rocky moments in the first few months of his tenure as leader, he has received mostly high marks from his colleagues for his stewardship of the Senate. And perhaps most importantly, Frist has brought calm and stability to the GOP Conference, which was under fire for Lott’s comments about then-Sen. Strom Thurmond’s 1948 segregationist presidential platform at the South Carolina Republican’s 100th birthday party.

“I hope Bill Frist runs,” said Sen. Lamar Alexander (R-Tenn.), the freshman lawmaker who twice unsuccessfully sought his party’s presidential nomination. “I hope he does run, because I think he is an unusually talented person and a very good experienced public servant.”

In addition to his legislative skills, Frist also has built up a national network of donors, a list he compiled as head of the Senate Republicans’ campaign arm in 2002. As National Republican Senatorial Committee chairman, Frist is widely credited with helping the GOP return to power after Democrats seized it in June 2001 when Sen. Jim Jeffords (I-Vt.) abandoned the Republican Party.

“He used it to his own great advantage,” a Republican strategist said in describing Frist’s tenure at the NRSC. “But it was also to everybody’s benefit.”

Frist has demonstrated he knows how to tap into this financial resource. In the first six months of this year, the Majority Leader raised a whopping $1.4 million for his leadership political action committee that he will likely dole back out to colleagues and gain valuable chits if he runs in 2008.

For his part, Frist claimed that for “right now” seeking the 2008 GOP nomination “is not my goal.” But he acknowledged that people are talking about his future candidacy.

“That is just because I occupy this position of Majority Leader,” he said in a matter-of-fact manner.

Frist is not the only person Republican insiders are fingering as future presidential candidates. Sens. John McCain (Ariz.) and Chuck Hagel (Neb.), two outspoken critics of the Republican establishment, are likely to toe the nomination waters, several Senators and aides predicted.

McCain battled Bush for the GOP nomination in 2000 and won the New Hampshire primary, but he eventually bowed out of the race after Bush’s win in the South Carolina primary made his nomination inevitable. Republican insiders view Hagel, a vocal Republican on foreign affairs issues, as very interested in a 2008 campaign.

McCain said he has “not given any contemplation to that scenario” of running for president again and chose to reference his much-discussed health when answering the question.

“I would be curious as to whether I live that long,” he deadpanned. His allies, however, are quick to note that if he were to run in ’08, he would be just three years older than President Ronald Reagan was when the Californian won his first term.

Senate Republican Conference Chairman Rick Santorum (Pa.), Conference Vice Chairwoman Kay Bailey Hutchison (Texas), NRSC Chairman George Allen (Va.) and Sen. Elizabeth Dole (N.C.) are also expected to take a long, hard look at a presidential campaign, Republican sources said.

For Democrats, Sen. Hillary Rodham Clinton (N.Y.) is the early favorite for her party’s 2008 presidential nomination. In fact, Clinton is so popular among Democratic voters that she would vault to frontrunner status if she chose to enter the 2004 presidential contest.

“You have got to look at Hillary because of her connections and the process of getting the nomination,” said a Senate Democratic leader, speaking on the condition of anonymity. “I would have to put her the odds-on favorite.”

Like Frist, Clinton is a prodigious fundraiser. She has attracted a loyal, national following that began when she became first lady in 1993 and has continued to grow since she was elected to the Senate in 2000. In the first six months of 2003, Clinton raised $718,000 for her leadership PAC.

In addition, Clinton is helping to raise money for Democrats as she crisscrosses the country to promote her memoir, “Living History.” She is considered to be the top draw on the fundraising circuit, according to top Democratic aides and strategists.

The 2008 Democratic primary field is also expected to include some combination of the quartet of Democratic Senators seeking the 2004 nomination: Sens. John Edwards (N.C.), Bob Graham (Fla.), John Kerry (Mass.) and Joe Lieberman (Conn.). Senate Minority Leader Tom Daschle (S.D.) and Sen. Evan Bayh (Ind.) are two other Democrats who people think will explore White House runs. Both men ruled out runs for the 2004 nomination — Bayh early on in the process, while Daschle chose to skip the 2004 contest in early January.

“Senator Daschle is relatively young and he remains a national Democrat with a significant base,” said Senate Democratic Policy Committee Chairman Byron Dorgan (N.D.).

Bayh, referencing his father’s (Indiana Democratic Sen. Birch Bayh) loss to Dan Quayle in 1980, said his sole focus is to get re-elected in 2004.

“I am not going to take anything for granted,” Bayh said. “I remember my father’s last campaign, which ended up being an unpleasant surprise, so I am sticking to my knitting.”

But Bayh would not rule out a future presidential run and said “there will be plenty of time to speculate about the future.”

Still the Indiana Democrat noted, “It is quite possible we could have a Democratic president running for re-election.”

Alexander, citing the amount of work he put into his 1995 campaign, including attending 250 fundraisers, walking across New Hampshire and visiting Iowa 80 times, said he has concluded it takes a special person to want to run for president.

“It takes an unusual sense of purpose and a certain amount of irrationality because … the difficulty of it and the complexity of it is exhausting,” said Alexander, who noted he plans to support Frist should the Majority Leader seek the nomination.