Deborah Ross seemed very much a long shot this year when she began her campaign against Senator Richard Burr, the powerful North Carolina Republican who is chairman of the Senate Select Committee on Intelligence. How could Ms. Ross, a Democrat, weather the inevitable Republican onslaught that she was “Radical Ross” — the “extreme liberal” who had been executive director of the state chapter of the American Civil Liberties Union? Political handicappers rated her background as prime fodder for Republican attack ads.

But Ms. Ross, who also served 10 years in the state legislature, proved to be a sharper politician than expected. She has stood up strongly in the crossfire, never apologizing for her liberal record. She built a positive identity with the public to a point that polls now show her in a tight race with Senator Burr, whose campaign has been somnolent and never seemed to take Ms. Ross seriously.

Republican shock and Democratic pride are palpable in North Carolina now that Ms. Ross has become pivotal in the Democrats’ hope to take control of the Senate from the Republicans led by Mitch McConnell, who has made it his mission to block President Obama’s initiatives. Electing Ms. Ross, a superior candidate, could help change the ossified, obstructionist state of Senate politics.

The contrast between the two candidates could not be sharper. Mr. Burr is a quiet party wheel horse whose career in the House and the Senate has been supported by significant campaign donations from the fossil fuel and nuclear energy industries. He was appointed to Donald Trump’s national security advisory panel, but, like many Republican incumbents, he has been hemming and hawing about Mr. Trump’s demoralizing candidacy. “I take him at his word,” the senator said rather meekly after Mr. Trump denied that he had ever committed the sexual assaults on women that he bragged about in the “Access Hollywood” tape. Mr. Burr has been trying to convince voters that Hillary Clinton’s “lack of judgment” is worse than Mr. Trump’s.