Neither is ahead in any polls, though Paul has made his mark on the campaign with big fundraising numbers and 10 percent finishes in early primary states. Given Paul's libertarian bent, he's unlikely to endorse any of the other Republicans running. In 2004, Kucinich waited until the last moment -- literally just short of the Democratic National Convention -- to endorse Sen. John Kerry's bid.
This has been a particularly packed election cycle, but the halls of Congress are littered with candidates past.
On the Democratic side, there are also the two men from Massachusetts.
Sen. Ted Kennedy, D-Mass., contemplated several bids and finally ran for president in 1980 against the incumbent Democrat, Jimmy Carter, but fell short in that primary battle.
John Kerry, Kennedy's junior Senate colleague, lost in a close contest in 2004 against President Bush. Kerry's running mate in 2004, former Sen. John Edwards of North Carolina, is at it again this year but lags in the polls.
Across the aisle, Sen. Lamar Alexander, R-Tenn., ran twice -- first in 1996, when he lasted until Super Tuesday, and most recently in 2000, when he drove around the country in a Ford Explore but didn't make it to the first primary.
Alexander, who was governor of Tennessee for much of the 1980s and secretary of education under President George H.W. Bush, did not come to the Senate until after his two failed bids.
Sen. Hillary Clinton, D-N.Y., is far from the first woman to aspire to the Oval Office -- she's not even the first political spouse to attempt the feat.
Sen. Elizabeth Dole, R-N.C., made a run in 2000, after her husband, former Sen. Bob Dole, made runs in 1992 and 1996. In the latter instance, Dole won the Republican nomination but lost the general election to former President Bill Clinton.
Mrs. Dole pulled out of the 2000 race in October, before the first primary, winning her Senate seat two years later.
Neither Senate Majority Leader Harry Reid nor Minority Leader Mitch McConnell has run for president, but both of their predecessors, Sens. Bill Frist of Tennessee and Tom Daschle of South Dakota seriously considered runs.
Neither did but both have now left the Senate: Frist by choice in 2006, Daschle by the choice of South Dakota voters in 2004.
And while there are any number of Congress members to be found roaming the halls after a failed White House bid, only one, John Quincy Adams, returned to Congress after actually serving as president.
Adams, the first son to win his father's old job, went from the House to the White House but returned again to Congress after losing re-election to Andrew Jackson.
Even according to his White House biography, Adams seems more fond of Congress than the executive branch.
It is reported there that in 1848, Adams "collapsed on the floor of the House from a stroke and was carried to the speaker's room, where two days later he died."