On November 6, after a grueling two-year campaign between competing political visions, Washington State voters went to the polls and overwhelmingly cast their ballots for Democrats. President Barack Obama won by 15 points; US senator Maria Cantwell by 20. Democrats won 9 of 10 partisan statewide races, including Governor-elect Jay Inslee, who carved out a three-point win against Rob McKenna, the two-term Republican attorney general and one-time odds-on favorite.

In Olympia, Republicans had boldly vowed to retake control of the state senate and to make a serious play for the state house. They did neither. As much as our wiser-than-thou editorial boards lament the Democrats' ongoing one-party control, that was the will of the people as expressed at the polls.

And yet, when the legislature convenes for a new session this January, any chance of enacting a Democratic agenda could teeter on the whim of a single state senator.

That's because Senator Rodney Tom, who was first elected as a Republican in 2002 and then switched parties to run as a Democrat in 2006, is now threatening to caucus with the Republicans again. "I think you can lead the senate in a more nonpartisan manner," says Senator Tom (?-Medina). By caucusing with the GOP, Tom could hand GOPers the very senate majority they couldn't win at the polls, while landing Tom in the agenda-setting role of majority leader.

"There's a million ways that this scenario plays out," says Tom.

Maybe. But the most obvious one is all too familiar: a repeat of last session, when Tom defected from Democrats in the eleventh hour to help Republicans seize control of key budget votes.

But Tom's about-face would bring much more far-reaching consequences in 2013: Starting in January, the legislature will consider a revenue package necessary to provide the billions more for K–12 education that was mandated under the state supreme court's McCleary decision. Also on the agenda: the Reproductive Parity Act that died when the Republicans and their "Roadkill Caucus" collaborators seized control of the senate last spring, and the Obamacare-sanctioned Medicaid expansion that proved a defining issue in the governor's race, which would extend health insurance to some 330,000 low-income Washingtonians. All this and more could languish under the divided government that Tom is proposing.

With a razor-close race in Vancouver's 17th Legislative District edging Republican as uncounted ballots run out—leaving Democrats with a bare 26–23 advantage—all that stands between Republicans and their obstructionist agenda is a mere two-vote swing in the senate. One of those votes is Tom's. The other belongs to conservative Democratic senator Tim Sheldon of Potlatch, an old-timey throwback to the days when political pork mattered more than political party.

It was Tom and Sheldon who joined Democratic senator Jim Kastama (who is now retiring) in a rarely used parliamentary procedure during the closing days of the last legislative session, dramatically seizing control of the senate and handing it to the Republicans. The goal was to impose a more austere state budget (Tom prefers the words "responsible" and "sustainable"), but with the Democrat-controlled house standing firm, this self-proclaimed "bipartisan" majority succeeded in fostering more bitterness than bipartisanship.

A monthlong special session ended with a budget that was not much different from the one the Democrats had originally proposed, save for a few face-saving concessions and an all-new bag of accounting tricks.

"When the house is controlled by the Democrats and there's a Democrat in the governor's mansion," asks political consultant Dean Nielsen, "what is Tom really accomplishing? Nothing."

And that's the most likely scenario should Tom carry through on his threat: a divided government that mires Olympia in partisan gridlock. But here's the bigger risk: The party that controls the senate controls who gets to chair the committees, and those chairs determine which legislation gets to the floor for a vote. A Republican majority could block any bill it wanted.

"We've been talking about this long before the election," says Tom, "that we would like to see the senate run a little more moderate." But the voters didn't cooperate. Tom acknowledges that Seattle's Ed Murray (D-43) is the undisputed leader of the Democratic caucus, having been elected majority leader by acclamation on November 13. So what makes Tom think that he knows better than both the caucus that elected Murray and the voters who put these Democratic senators in control?

"Ed is from a very liberal district [and] is quite possibly running for mayor of Seattle," worries Tom, "and I don't know if that is in the best interest of the state."

If Tom does decide to caucus with the Republicans, he may well be empowered as one of the most powerful lawmakers—or obstructionists—in Olympia. But there is a downside for Tom. In blue Washington, his suburban district is growing more and more Democratic every election.

"If they vote me in, great; if they vote me out, I'm fine with that, too," says Tom. We'll see if he's still singing that tune after Election Day 2014.