"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 FrankenFranken to make first public appearance since resignation Overnight Cybersecurity: Fallout from Comey memos | IG reportedly investigating memos over classified info | DNC sues Russia, Trump campaign | GOP chair blasts FDIC over data security Why Smokin' Joe leads the pack of 2020 Democratic hopefuls MORE (D-Minn.) and Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups 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.