"But I have you on my side.  Let’s show the SuperPACs that I’ve got the grassroots with me – and that they’d better pick a fight with someone else," Merkley adds. 

No Republican candidates have yet emerged for the seat. But Merkley won in 2008 by only 3 percentage points, and Oregon Republicans have indicated they plan to make a push to defeat him in 2014.

That's likely why the Senator started fundraising efforts for his reelection campaign as early as April of this year. President Obama won Oregon with 54 percent of the vote, but a poll conducted earlier this month indicated 44 percent of Oregonians approve of the job he's done as senator. That's an improvement from the 37 percent who approved of his job in a June poll, but the newest poll indicates he could be vulnerable to a challenge: He leads every hypothetical Republican contender, but only leads former GOP Sen. Gordon Smith by 4 percentage points, and Rep. Greg Walden (R) by 5 percentage points.

Merkley had a little less than $500,000 cash on hand as of the end of November, far less than some of his fellow freshman Senators that could face challenges in 2014, including Sens. Al FrankenAlan (Al) Stuart FrankenPawlenty opts out of Senate run in Minnesota EMILY’s List president: Franken did 'right thing for Minnesota' Dem pledges to ask all court nominees about sexual harassment history under oath MORE (D-Minn.) and Mark UdallMark Emery UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' MORE (D-Colo.), both of whom have more than $1 million cash on hand.

--This post was updated to reflect the most recent polling numbers.